Let me attempt to make a long story short.
In koine Greek (the Greek of the New Testament and of the Septuagint--the Greek translation of the Hebrew Bible) the standard word for "glory" is kleos. It is used for the fame, renown, or reputation someone can gain as a result of their notoriety. The Septuagint translators chose not to use this word to translate the Hebrew kabad, most often translated "glory," but almost always only in reference to God.
They chose instead the word doxa, which, as a noun in ancient Greek, usually means an "opinion," "belief," and sometimes, "reputation" (it still hads this sense in our English word "orthodoxy"). One can guess (no one knows for sure) that doxa was chosen to have a fresh word not tied up in definitions about the "glory" of human beings. Nevertheless, in choosing the word doxa, they fundamentally changed its meaning, not only from "opinion" to "glory," but from something totally subjective to one, in its use to describe God's nature and being, totally objective.
"Glory," whatever Hebrew or Greek word it is translating, is difficult to define. I think it is best to be thought of as a word that describes, as I just said, the very nature and being of God. It can have the connotation of "light," "wonder," "mystery," "awe," "perfection," just about any word you could find to describe God.
The New Testament, by the way, follows the use of the Septuagint. The original word for "glory," kleos, appears only one time, in 1 Peter 2:20. It was translated as "glory" in the King James Version, but not since then. Doxa appears at least 150 times.
The New Testament takes the word further, however, and ascribes it to humankind, but only in the sense of our being able to participate in the divine glory, or, as Paul says in 2 Cor 3:18 (where the title of this blog comes, through its use by Charles Wesley),