What is the "glory of God" in Romans 3:23?

In the original Greek, the word sin was an archery term, and literally means "to miss the mark" - to miss the bullseye or perfection.
However far a person hit outside the bullseye of perfection was referred to as one's "sin mark".

The word “sin”, as it appears in the Bible, comes from the Greek word Hamartia or the Hebrew word Hata, which both mean “to miss the mark” or “flawed”. The word was used in archery and spear throwing. When a person missed the center of the target, they erred, or “hamartia-ed”. So, as this applies to sin, we should ask, “What is the mark that we are missing? And the answer is: The holy righteous perfection of God. When it comes to missing that mark, we’re not just veering off a little and barely missing the bull’s-eye… on our way to the range, we took a wrong turn and drove off a cliff!

https://www.patheos.com/blogs/e2medianetwork/2016/11/wwutt-sin-means-to-miss-the-mark/

It's important to understand that this is totally different than a mistake or sin of ignorance. It's a willful choice to disobey God - an act of rebellion.
James said - "To him who knows to do right and does not do it - to him it is sin." https://www.gospelproject.com/sin-missing-mark-video/

"One aspect of sin is missing the mark of God’s standards set for humanity. At first blush, missing the mark sounds like it could be referring to a simple mistake or an unintentional error. But missing the mark actually refers to falling short of God’s glory through conscious choosing of sin. So, is it appropriate to refer to sin as a failure on our part to live according to God’s standards? Sure, but here’s the thing: we have to recognize that this failure is intentional. We miss the mark when we deliberately choose to cast aside God’s intention for us.

As the Bible says we are created in His image, In His Likeness, we can also say In His Glory.

Using the context above, we can say to fall short means we are no more at that sinless or holy state(God's glory) in which Adam was first created.

A respected preacher from the last century referred to God's glory as "the outshining of God's presence." That definition is a good one. Well, at least it is a good start for a subject so profoundly deep.

The expression "the outshining of God's presence" reminds me of one of the apostle John's descriptions of God:

This is the message we have heard from him and declare to you: God is light; in him there is no darkness at all.

In what sense is God light, you may ask. Before I attempt an answer to the question, Saint Paul's words in his first letter to Timothy lay the foundation for an answer:

God, the blessed and only Ruler, the King of kings and Lord of lords, who alone is immortal and who lives in unapproachable light, whom no one has seen or can see. To him be honor and might forever. Amen (6:15b-16 NIV).

The Light that IS God is not light in the usual sense. The sense in which God's image-bearers use the term refers to the natural light, created by God (Genesis 1) and provided by the sun, moon, and stars. The outshining of God's presence is a different kind of light altogether. The Light that is God is inherent to God's perfections, particularly his holiness.

When Moses asked God to show him His glory, this is what God said to Moses:

“I will cause all my goodness to pass in front of you, and I will proclaim my name, the LORD, in your presence. I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion. But,” he said, “you cannot see my face, for no one may see me and live.”

Then the Lord said, “There is a place near me where you may stand on a rock. When my glory passes by, I will put you in a cleft in the rock and cover you with my hand until I have passed by. Then I will remove my hand and you will see my back; but my face must not be seen” (Exodus 33).

When the apostle Paul tells us that we all fall short of the glory of God, he is saying every human being who ever lived or ever will live is separated from God by a great chasm which can never be bridged, unless God graciously bestows on them both the gift of forgiveness and the gift of eternal life. Clothed in the righteousness of Christ, sinners become new creations in him. Moreover, they have the privilege of being lights in the world. Their light is not inherent to them, but it is a reflected light that originates in the God who IS light.

In conclusion, at one time Moses' face reflected God's eternal and perfect light. We read in Exodus,

It came about when Moses was coming down from Mount Sinai (and the two tablets of the testimony were in Moses’ hand as he was coming down from the mountain), that Moses did not know that the skin of his face shone because of his speaking with Him. So when Aaron and all the sons of Israel saw Moses, behold, the skin of his face shone, and they were afraid to come near him (34:29-30 NASB).

We know that the outshining of God's presence as it was seen in a reflected form on Moses' face faded (see 2 Corinthians 3:7-17). That is because Moses, as with all of humankind, fell short of the glory of God. Today, however, because of the New Covenant in Jesus's blood, all Christians, as the apostle Paul observed, are

. . . with unveiled face, beholding as in a mirror the glory of the Lord, [and] are being transformed into the same image from glory to glory, just as from the Lord, the Spirit.