The first definition of grace provided over at Merriam-Webster Online is actually a pretty good one:
grace: 1 a: unmerited divine assistance given humans for their regeneration or sanctification b: a virtue coming from God c: a state of sanctification enjoyed through divine grace
However, notice that this first definition has three subentries (a, b, and c) and that there are seven alternate definitions for the word grace. There are also ten additional entries in a scroll box on the grace page.
With all of those definitions rattling around, it’s no wonder grace is hard to define. I grew up in a church that has a lot to say about grace. I’ve often thought it strange that, even though the idea of God’s grace is central to my church’s teaching, it has taken me a long time to be able to talk about it coherently.