Obviously the definitions will vary wildly depending on who you ask, but there are are baseline definitions that most will agree on.

Catholic Answers (which strictly adheres to the Catechism of the Catholic Church) defines a miracle as an extraordinary sensible effect wrought by God that surpasses the power and order of created nature.

This seems to be a pretty good definition. I would define it (as a non-denominational Christian), as an event that defies the laws of nature.

I don't know how or why any Christians would object to this definition.

Is there a standard definition of the word 'miracle' in Christianity?

There is probably no one standard definition for the word miracle; at least not for what constitutes a miracle in itself because of the fact that Christians can not always agree on to what is a miracle.

For example, Catholics believe that the act of consecration of bread and wine at every Catholic mass is a miracle (Transubstantiation). Some Christian denominations will not recognize this as such.

Although not a classical definition, Wikipedia seems also to have generally open-ended definition for a miracle:

A miracle is a supernatural event that seems inexplicable by natural or scientific laws. In various religions, a phenomenon that is characterized as miraculous is often attributed to the actions of a supernatural being, (especially) a deity, a magician, a miracle worker, a saint, or a religious leader

The Catholic Church believes miracles are works of God, either directly, or through the prayers and intercessions of a specific saint or saints. There is usually a specific purpose connected to a miracle, e.g. the conversion of a person or persons to the Catholic faith or the construction of a church desired by God. The Church says that it tries to be very cautious to approve the validity of putative miracles. The Catholic Church says that it maintains particularly stringent requirements in validating the miracle's authenticity. The process is overseen by the Congregation for the Causes of Saints.

For a majority of Evangelical Christians, biblicism ensures that the miracles described in the Bible are still relevant and may be present in the life of the believer. Healings, academic or professional successes, the birth of a child after several attempts, the end of an addiction, etc., would be tangible examples of God's intervention with the faith and prayer, by the Holy Spirit. In the 1980s, the neo-charismatic movement re-emphasized miracles and faith healing. In certain churches, a special place is thus reserved for faith healings with laying on of hands during worship services or for campaigns evangelization. Faith healing or divine healing is considered to be an inheritance of Jesus acquired by his death and resurrection.

The most basic and classic definition of a miracle would be as follows:

Miracle

Definition

A sensibly perceptible effect, surpassing at least the powers of visible nature, produced by God to witness to some truth or testify to someone's sanctity. (Etym. Latin miraculum, miracle, marvel; from mirari, to wonder.)

How to interpret this definition will vary according to individuals and denominations.

The following lengthy article on miracles from the Catholic Encyclopedia explains well the in-depth viewpoint according to Catholicism:

Miracle

In general, a wonderful thing, the word being so used in classical Latin; in a specific sense, the Latin Vulgate designates by miracula wonders of a peculiar kind, expressed more clearly in the Greek text by the terms terata, dynameis, semeia, i.e., wonders performed by supernatural power as signs of some special mission or gift and explicitly ascribed to God.

These terms are used habitually in the New Testament and express the meaning of miraculum of the Vulgate. Thus St. Peter in his first sermon speaks of Christ as approved of God, dynamesin, kai terasin kai semeiois (Acts 2:22) and St. Paul says that the signs of his Apostleship were wrought, semeiois te kai terasin kai dynamesin (2 Corinthians 12:12). Their united meaning is found in the term erga i.e., works, the word constantly employed in the Gospels to designate the miracles of Christ. The analysis of these terms therefore gives the nature and scope of the miracle.

Nature

(1) The word terata literally means "wonders", in reference to feelings of amazement excited by their occurrence, hence effects produced in the material creation appealing to, and grasped by, the senses, usually by the sense of sight, at times by hearing, e.g., the baptism of Jesus, the conversion of St. Paul. Thus, though the works of Divine grace, such as the Sacramental Presence, are above the power of nature, and due to God alone, they may be called miraculous only in the wide meaning of the term, i.e., as supernatural effects, but they are not miracles in the sense here understood, for miracles in the strict sense are apparent. The miracle falls under the grasp of the senses, either in the work itself (e.g. raising the dead to life) or in its effects (e.g., the gifts of infused knowledge with the Apostles). In like manner the justification of a soul in itself is miraculous, but is not a miracle properly so called, unless it takes place in a sensible manner, as, e.g., in the case of St. Paul.

The wonder of the miracle is due to the fact that its cause is hidden, and an effect is expected other than what actually takes place. Hence, by comparison with the ordinary course of things, the miracle is called extraordinary. In analyzing the difference between the extraordinary character of the miracle and the ordinary course of nature, the Fathers of the Church and theologians employ the terms above, contrary to, and outside nature. These terms express the manner in which the miracle is extraordinary.

A miracle is said to be above nature when the effect produced is above the native powers and forces in creatures of which the known laws of nature are the expression, as raising a dead man to life, e.g., Lazarus (John 11), the widow's son (1 Kings 17). A miracle is said to be outside, or beside, nature when natural forces may have the power to produce the effect, at least in part, but could not of themselves alone have produced it in the way it was actually brought about. Thus the effect in abundance far exceeds the power of natural forces, or it takes place instantaneously without the means or processes which nature employs. In illustration we have the multiplication of loaves by Jesus (John 6), the changing of water into wine at Cana (John 2) — for the moisture of the air by natural and artificial processes is changed into wine — or the sudden healing of a large extent of diseased tissue by a draught of water. A miracle is said to be contrary to nature when the effect produced is contrary to the natural course of things.

The term miracle here implies the direct opposition of the effect actually produced to the natural causes at work, and its imperfect understanding has given rise to much confusion in modern thought. Thus Spinoza calls a miracle a violation of the order of nature (proeverti, "Tract. Theol. Polit.", vi). Hume says it is a "violation" or an "infraction", and many writers — e.g., Martensen, Hodge, Baden-Powell, Theodore Parker — use the term for miracles as a whole. But every miracle is not of necessity contrary to nature, for there are miracles above or outside nature.

Again, the term contrary to nature does not mean "unnatural" in the sense of producing discord and confusion. The forces of nature differ in power and are in constant interaction. This produces interferences and counteractions of forces. This is true of mechanical, chemical, and biological forces. So, also, at every moment of the day I interfere with and counteract natural forces about me. I study the properties of natural forces with a view to obtain conscious control by intelligent counteractions of one force against another. Intelligent counteraction marks progress in chemistry, in physics — e.g., steam locomotion, aviation — and in the prescriptions of the physician. Man controls nature, nay, can live only by the counteraction of natural forces. Though all this goes on around us, we never speak of natural forces violated. These forces are still working after their kind, and no force is destroyed, nor is any law broken, nor does confusion result. The introduction of human will may bring about a displacement of the physical forces, but no infraction of physical processes.

Now in a miracle God's action relative to its bearing on natural forces is analogous to the action of human personality. Thus, e.g., it is against the nature of iron to float, but the action of Eliseus in raising the axe-head to the surface of the water (2 Kings 6) is no more a violation, or a transgression, or an infraction of natural laws than if he raised it with his hand. Again, it is of the nature of fire to burn, but when, e.g., the Three Children were preserved untouched in the fiery furnace (Daniel 3) there was nothing unnatural in the act, as these writers use the word, any more than there would be in erecting a dwelling absolutely fireproof. In the one case, as in the other, there was no paralysis of natural forces and no consequent disorder.

The extraordinary element in the miracle — i.e. an event apart from the ordinary course of things; enables us to understand the teaching of theologians that events which ordinarily take place in the natural or supernatural course of Divine Providence are not miracles, although they are beyond the efficiency of natural forces. Thus, e.g., the creation of the soul is not a miracle, for it takes place in the ordinary course of nature. Again, the justification of the sinner, the Eucharistic Presence, the sacramental effects, are not miracles for two reasons: they are beyond the grasp of the senses and they have place in the ordinary course of God's supernatural Providence.

(2) The word dynamis, "power" is used in the New Testament to signify:

• the power of working miracles, (en dymamei semeion — Romans 15:19);

• mighty works as the effects of this power, i.e., miracles themselves (al pleistai dynameis autou — Matthew 11:20) and expresses the efficient cause of the miracle, i.e., Divine power.

Hence the miracle is called supernatural, because the effect is beyond the productive power of nature and implies supernatural agency. Thus St. Thomas teaches: "Those effects are rightly to be termed miracles which are wrought by Divine power apart from the order usually observed in nature" (Contra Gent., III, cii), and they are apart from the natural order because they are "beyond the order or laws of the whole created nature" (ST I-II:113:10). Hence dynamis adds to the meaning of terata by pointing out the efficient cause. For this reason miracles in Scripture are called "the finger of God" (Exodus 8:19, Luke 11:20), "the hand of the Lord" (1 Samuel 5:6), "the hand of our God" (Ezra 8:31). In referring the miracle to God as its efficient cause the answer is given to the objection that the miracle is unnatural, i.e., an uncaused event without meaning or place in nature. With God as the cause, the miracle has a place in the designs of God's Providence (Contra Gent. III, xcviii). In this sense — i.e., relatively to God — St. Augustine speaks of the miracle as natural (City of God XXI.8). An event is above the course of nature and beyond its productive powers:

• with regard to its substantial nature, i.e., when the effect is of such a kind that no natural power could bring it to pass in any manner or form whatsoever, as e.g., the raising to life of the widow's son (Luke 7), or the cure of the man born blind (John 9). These miracles are called miracles as to substance (quoad substantiam).

With regard to the manner in which the effect is produced i.e., where there may be forces in nature fitted and capable of producing the effect considered in itself, yet the effect is produced in a manner wholly different from the manner in which it should naturally be performed, i.e., instantaneously, by a word, e.g., the cure of the leper (Luke 5). These are called miracles as to the manner of their production (quoad modum).

God's power is shown in the miracle:

• directly through His own immediate action or mediately through creatures as means or instruments.

In the latter case the effects must be ascribed to God, for He works in and through the instruments; "Ipso Deo in illis operante" (Augustine, City of God X.12). Hence God works miracles through the instrumentality

• of angels, e.g., the Three Children in the fiery furnace (Daniel 3), the deliverance of St. Peter from prison (Acts 12);

• of men, e.g., Moses and Aaron (Exodus 7), Elias (1 Kings 17), Eliseus (2 Kings 5), the Apostles (Acts 2:43), St. Peter (Acts 3:9), St. Paul (Acts 19), the early Christians (Galatians 3:5).

In the Bible also, as in church history, we learn that animate things are instruments of Divine power, not because they have any excellence in themselves, but through a special relation to God. Thus we distinguish holy relics, e.g., the mantle of Elias (2 Kings 2), the body of Eliseus (2 Kings 13), the hem of Christ's garment (Matthew 9), the handkerchiefs of St. Paul (Acts 19:12); holy images, e.g., the brazen serpent (Numbers 21) holy things, e.g., the Ark of the Covenant, the sacred vessels of the Temple (Daniel 5); holy places, e.g., the Temple of Jerusalem (2 Chronicles 6:7), the waters of the Jordan (2 Kings 5), the Pool of Bethsaida (John 5). Hence the contention of some modern writers, that a miracle requires an immediate action of Divine power, is not true. It is sufficient that the miracle be due to the intervention of God, and its nature is revealed by the utter lack of proportion between the effect and what are called means or instruments.

The word semeion means "sign", an appeal to intelligence, and expresses the purpose or final cause of the miracle. A miracle is a factor in the Providence of God over men. Hence the glory of God and the good of men are the primary or supreme ends of every miracle. This is clearly expressed by Christ in the raising of Lazarus (John 11), and the Evangelist says that Jesus, in working His first miracle at Cana, "manifested his glory" (John 2:11). Therefore the miracle must be worthy the holiness, goodness, and justice of God, and conducive to the true good of men. Hence they are not performed by God to repair physical defects in His creation, nor are they intended to produce, nor do they produce, disorder or discord; do they contain any element which is wicked, ridiculous, useless, or unmeaning. Hence they are not on the same plane with mere wonders, tricks works of ingenuity, or magic. The efficacy, usefulness, purpose of the work and the manner of performing it clearly show that it must be ascribed to Divine power. This high standing and dignity of the miracle is shown, e.g., in the miracles of Moses (Exodus 7-10), of Elias (1 Kings 18:21-38), of Eliseus (2 Kings 5). The multitudes glorified God at the cure of the paralytic (Matthew 9:8), of the blind man (Luke 18:43), at the miracles of Christ in general (Matthew 15:31, Luke 19:37), as at the cure of the lame man by St. Peter (Acts 4:21). Hence miracles are signs of the supernatural world and our connection with it.

In miracles we can always distinguish secondary ends, subordinate, however, to the primary ends. Thus

• they are evidences attesting and confirming the truth of a Divine mission, or of a doctrine of faith or morals, e.g., Moses (Exodus 4), Elias (1 Kings 17:24). For this reason the Jews see in Christ "the prophet" (John 6:14), in whom "God hath visited his people" (Luke 7:16). Hence the disciples believed in Him (John 2:11) and Nicodemus (John 3:2) and the man born blind (John 9:38), and the many who had seen the raising of Lazarus (John 11:45). Jesus constantly appealed to His "works" to prove that He was sent by God and that He is the Son of God, e.g., to the Disciples of John (Matthew 11:4), to the Jews (John 10:37). He claims that His miracles are a greater testimony than the testimony of John (John 5:36), condemns those who will not believe (John 15:24), as He praises those who do (John 17:8), and exhibits miracles as the signs of the True Faith (Mark 16:17). The Apostles appeal to miracles as the confirmation of Christ's Divinity and mission (John 20:31; Acts 10:38), and St. Paul counts them as the signs of his Apostleship (2 Corinthians 12:12).

Miracles are wrought to attest true sanctity. Thus, e.g., God defends Moses (Numbers 12), Elias (2 Kings 1), Eliseus (2 Kings 13). Hence the testimony of the man born blind (John 9:30 sqq.) and the official processes in the canonization of saints.

As benefits either spiritual or temporal. The temporal favours are always subordinate to spiritual ends, for they are a reward or a pledge of virtue, e.g. the widow of Sarephta (1 Kings 17), the Three Children in the fiery furnace (Daniel 3), the preservation of Daniel (Daniel 5), the deliverance of St. Peter from prison (Acts 12), of St. Paul from shipwreck (Acts 27). Thus semeion, i.e., "sign", completes the meaning of dynamis, i.e., "[Divine] power". It reveals the miracle as an act of God's supernatural Providence over men. It gives a positive content to teras, i.e., "wonder", for, whereas the wonder shows the miracle as a deviation from the ordinary course of nature, the sign gives the purpose of the deviation.

This analysis shows that

• the miracle is essentially an appeal to knowledge. Therefore miracles can be distinguished from purely natural occurrences. A miracle is a fact in material creation, and falls under the observation of the senses or comes to us through testimony, like any natural fact. Its miraculous character is known:

• from positive knowledge of natural forces, e.g., the law of gravity, the law that fire burns. To say that we do not know all the laws of nature, and therefore cannot know a miracle (Rousseau, "Lett. de la Mont.", let. iii), is beside the question, for it would make the miracle an appeal to ignorance. I may not know all the laws of the penal code, but I can know with certainty that in a particular instance a person violates one definite law.

From our positive knowledge of the limits of natural forces. Thus, e.g., we may not know the strength of a man, but we do know that he cannot by himself move a mountain. In enlarging our knowledge of natural forces, the progress of science has curtailed their sphere and defined their limits, as in the law of abiogenesis. Hence, as soon as we have reason to suspect that any event, however uncommon or rare it appear, may arise from natural causes or be conformable to the usual course of nature, we immediately lose the conviction of its being a miracle. A miracle is a manifestation of God's power; so long as this is not clear, we hould reject it as such.

Miracles are signs of God's Providence over men, hence they are of high moral character, simple and obvious in the forces at work, in the circumstances of their working, and in their aim and purpose. Now philosophy indicates the possibility, and Revelation teaches the fact, that spiritual beings, both good and bad, exist, and possess greater power than man possesses. Apart from the speculative question as to the native power of these beings, we are certain that God alone can perform those effects which are called substantial miracles, e.g., raising the dead to life,

• that miracles performed by the angels, as recorded in the Bible, are always ascribed to God, and Holy Scripture gives Divine authority to no miracles less than Divine;

• that Holy Scripture shows the power of evil spirits as strictly conditioned, e.g., testimony of the Egyptian magicians (Exodus 8:19), the story of Job, evil spirits acknowledging the power of Christ (Matthew 8:31), the express testimony of Christ himself (Matthew 24:24) and of the Apocalypse (Revelation 9:14). Granting that these spirits may perform prodigies — i.e., works of skill and ingenuity which, relatively to our powers, may seem to be miraculous — yet these works lack the meaning and purpose which would stamp them as the language of God to men.

Types of miracles

St. Thomas Aquinas distinguishes the three types of miracles, miracles being "works that are sometimes done by God outside the usual order assigned to things", in Summa Contra Gentiles, lib. 3 cap. 101 (from highest to lowest):

1. "those events in which something is done by God which nature never could do."
2. "those events in which God does something which nature can do, but not in this order"
3. "when God does what is usually done by the working of nature, but without the operation of the principles of nature."