What is attachment as per Buddhism?

Attachment is craving for sensation either in the form of direct sensual experience or a future situation or existance in which you can experience or be united with desirable sensations. Our mental reaction to either external stimuli or comparison of some mental models against what we sensed produce sensation. It is pleasant sensation that you are attached or crave for. The fact that we have friend is no attachment at all unless, where have some Perceived relationship in a metal outlook we create about the world and we react to the sensation this bring about. Each metal state resulting from the contact our our 6 faculties create sensation. We perceive these sedations as favourable, unfavourable or neutral and react accordingly with craving, aversion, or ignorance.

Letting go is giving up your Perception of sensual experiences as favourable, unfavourable and developing equanimity towards any sensation always realising they arise and passes away.

Love is a kind of attachment when you perceive some one in relation to you (myself, my friend, my wife, my child, etc.).

See:

• Pahāna Sutta,
• Vedana From Devotion to Direct Experience by S. N. Goenka

Also quotes from meditation teachers who have similar line of explanation:

A sensation appears, and liking or disliking begins. This fleeting moment, if we are unaware of it, is repeated and intensified into craving and aversion, becoming a strong emotion that eventually overpowers the conscious mind. We become caught up in the emotion, and all our better judgment is swept aside. The result is that we find ourselves engaged in unwholesome speech and action, harming ourselves and others. We create misery for ourselves, suffering now and in the future, because of one moment of blind reaction.

But if we are aware at the point where the process of reaction begins–that is, if we are aware of the sensation–we can choose not to allow any reaction to occur or to intensify… in those moments the mind is free. Perhaps at first these may be only a few moments in a meditation period, and the rest of the time the mind remains submerged in the old habit of reaction to sensations, the old round of craving, aversion, and misery. But with repeated practice those few brief moments will become seconds, will become minutes, until finally the old habit of reaction is broken, and the mind remains continuously at peace. This is how suffering can be stopped.

Source: Art of Living by S.N. Goenka

Once we understand the secrets of the kaya, we turn to the secrets of the vedana. The vedana have the highest power and influence over human beings, over all living things. My words will surprise you, nonetheless, the entire world - animals, humans, and all living beings – depends on the vedana. They all are under the power of the feelings. This sounds funny and unbelievable, so examine it for yourselves. It is a fact that we - our entire species – are being forced by the vedana to do their bidding. When there are sukha-vedana (pleasant feelings) we try to get more of those feelings. The pleasant feelings always pull the mind in a certain direction and condition certain kinds of activity. Dukkha-vedana (unpleasant, disagreeable feelings) affect the mind and influence life in the opposite direction, but still lead to all kinds of habitual responses. The mind struggles with them and turns them into problems that cause dukkha. The feelings have great power over what we do. The whole world is under the command of these vedana, although there may be other factors involved as well. For example, while tanha (craving) can control the mind, craving itself is first conditioned by feeling. Thus, the vedana have the strongest and most powerful influence over our entire mind. We ought to understand the secrets of vedana. (25)

I will say something at which you can laugh if you wish, "If we can master the vedana we will be able to master the world." We will be able to control the world, when we can control the feelings as we require. Then we could supervise the world as it so badly needs. Now, nobody is interested in controlling the vedana, so the world has gotten out of proper control. Have you seen all the crises and problems that arise constantly? The wars, the famines, the corruption, the pollution, all these things, are activities originating in our failure to control the feelings from the start. If we would control the feelings, then we could control the world. This is something you need to consider. (26)

If we speak in line with the Lord Buddha's words, we say that the causes of everything in the world are centered on the vedana. The myriad activities happen in the world because our feeling of the vedana forces us to desire, and then act out those desires. Even such beliefs as reincarnation and rebirth are conditioned by the vedana. If we believe, we travel around in the samsaric cycle, the cycles of birth and death, of heaven and hell. Everything originates in feeling. To control the vedana is to control the origin, the source, the birth place of all things. This is how necessary it is to understand these feelings correctly and comprehensively. Then, we will be able to control them and their secrets will not deceive us into doing anything foolish ever again. (27)

There are three main points to realize regarding the vedana. First, understand the vedana themselves, the things that cause feeling in the mind, that the mind feels. Second, know how the vedana condition the citta, the mind-heart. They stir up thoughts, memories, words, and actions. Know this concocting of the mind. Third, discover that we can control the mind by controlling the vedana, in the same way that the flesh-body is controlled by the regulation of the breath. Then we win be able to master the mind by correctly mastering the feelings which condition it. These three things make up the secrets of the vedana.

1. Understand the feelings themselves.
2. Know the things that condition the feelings.
3. Then, know how to control those things that condition the feelings, which is the same as controlling the feelings themselves. These are the three important things to understand about vedana. (28)

Since the first and second stages of practice both follow the same principle, it is helpful to compare the two. In the stage regarding the body, we find out what it is that conditions the flesh-body, and then we study that thing. We study that body-conditioner until we know it in great detail. We study how that thing conditions the body. Then, by regulating that thing, we can control the body. This is our way to make the body more calm and peaceful. As for the mind, its conditioner is the feelings. By controlling the vedana so that they do not condition or stir up the mind, or so that they condition the mind in a desirable way, we are able to calm the mind. This how the first stage regarding the kaya and the second stage regarding the vedana follow the same basic principle and are parallel in their method of practice.

Source: ANAPANASATI - MINDFULNESS WITH BREATHING Unveiling the Secrets of Life: a Manual for Serious Beginners by BUDDHADASA BHIKKHU

Love and attachment are two different things. Love is no separation between beings, two beings or all beings. Love is selfless, not selfish. Attachment can be selfish.

Love can be difficult to untangle from from attachment and understand what is what. Attachment, for example, is when a mother who won't let her child leave when it's time for them to leave because mother is clinging. To complicate matters, the child could be clinging as well, perhaps to the idea of being an adult and the mother maybe knows better or perhaps there both clinging in some unwholesome way. It takes wisdom.

If one with good concentration & moral intentions constantly contemplates one's own karma(actions) then one will slowly understand for certain what is wholesome and what isn't a wholesome action to perform.

Morality and concentration that supports contemplation of one's moment to moment experiences as they really are(with a practice like Vipassana), is how one can cultivate wisdom(like knowing the difference between attachment and love). Wisdom is what frees us from our suffering :)

According to the Wikipedia article Twelve Nidānas, the Pali word for Attachment is Upādāna (and that Wikpedia section has an unreferenced description of it).

That also links to a whole Wikipedia article on the subject, Upādāna, which says,

Upādāna and taṇhā are seen as the two primary causes of suffering. The cessation of clinging leads to Nirvana.

That statement in that article does have a lot of references.

However, I note that the Third Noble Truth in the Dhammacakkappavattana Sutta says "desire" and not "attachment". This commentary says,

Furthermore, bhikkhus, this is the dukkha·nirodha ariya·sacca: the complete virāga, nirodha, abandoning, forsaking, emancipation and freedom from that very taṇhā.

... where Taṇhā means thirst or desire.

Clearly, though, desire and attachment are related.

The Upadana Sutta starts with,

Dwelling at Savatthi. There the Blessed One said to the monks:

"In one who keeps focusing on the allure of clingable phenomena (or: phenomena that offer sustenance = the five aggregates), craving develops. From craving as a requisite condition comes clinging/sustenance.

I think that those two quoted sentences are saying:

• If you focus on what's alluring about something that you are or can become attached to, then you'll desire that thing
• If you desire something then you become attached to (cling to) it

In summary I think that if desire is like thirst then attachment is the object of that desire. For example you might think,

I am thirsty therefore I would like some water

... in which case you have attached to water as the object of your desire.

Similarly if you desire a person, then the desire is desire and the person (or more specifically your idea or 'reification' of that person) is the object of your desire and thus the focus of your attachment.

I suspect that attachment is also related to ego or sense of self, if you become attached to something then you identify with it, you think, "I desire this, I am the person who feels desire for this", etc. I say that because the Upadana Sutta above said that the "clingable phenomena" are the five aggregates; and the Anatta-lakkhana Sutta also talks about the five aggregates including the famous saying,

Any [of each of the five aggregates] must, with right understanding how it is, be regarded thus: 'This is not mine, this is not I, this is not my self

Wikipedia's Manifestations of clinging says,

To distinguish craving from clinging, Buddhaghosa uses the following metaphor:[14]

"Craving is the aspiring to an object that one has not yet reached, like a thief's stretching out his hand in the dark; clinging is the grasping of an object that one has reached, like the thief's grasping his objective.... [T]hey are the roots of the suffering due to seeking and guarding."

Although I may be wrong (perhaps I have misunderstood what 'attachment means), but I might not quite agree with that metaphor: because I think it's possible to be attached even to an object that you don't or cannot grasp.

For example if you were to think, "If only I had that person, or, if only I had that thing, then I'd be happy", then I think that's not only generalized desire (for an object) but also specific attachment (to a specific object): even when and even though you don't or cannot have/grasp/own/reach that object.

Presumably though you can be said to "have it" in the sense that "you have the idea of it".