In classical Greek, charis referred to a gift given freely and generously because the giver found pleasure in giving it. It had three dimensions: the gift itself, the disposition of the giver, and the gratitude of the recipient — all three were contained in the one word. Aristotle described charis as a favor given not in return for anything and not so that the giver would gain anything, but purely for the benefit of the one who receives it. The Latin equivalent was gratia, which gave English both "grace" and "gratitude" — both sides of the transaction compressed into related words.
The Septuagint used charis to translate the Hebrew chen, which referred to the favor of a superior toward someone who had no claim on that favor. When Noah found "grace in the eyes of the LORD" (Genesis 6:8), it was not because Noah had accumulated sufficient merit. It was because God looked toward him with unearned goodwill. The concept carries this flavor throughout the Old Testament: it is always the initiative of the one with power, extended freely toward the one without.
Paul's use of charis in Ephesians 2:8 is the classic formulation: "For by grace are ye saved through faith; and that not of yourselves: it is the gift of God." The word "gift" there is doron, and it doubles down on what charis already implies — that no contribution from the recipient made any part of it happen. But Paul goes further in Romans 11:6: "And if by grace, then is it no more of works: otherwise grace is no more grace." Grace cannot be partially grace. The moment any element of earning enters the picture, the word no longer applies. It is an all-or-nothing concept by definition.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.