Charis is the Greek word for grace — favour freely given, with no thought of return. In everyday Greek it meant a gift or a kindness that delighted the giver; in the New Testament it becomes the towering word for God's unearned goodness toward sinners. By charis we are saved, through faith; and charis, by definition, cannot be worked for — if it were earned, it would no longer be grace.
Charis is related to the Greek word for joy, and the two belong together: grace is the gift that produces gladness. It is also the root of eucharist (thanksgiving) — the grateful response grace calls forth. The whole Christian life runs on charis from start to finish: saved by grace, sustained by grace (my grace is sufficient for thee), and welcomed at the throne of grace to find grace to help in time of need.
Charis in Scripture
For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: