Where did the word "Purgatory" come from? It is NOT in the Bible.?

Today, Thanksgiving, a few of my non-Catholic friends and I were discussing Purgatory. They do not belive in it. I do, but I felt awful because I could not show where the word Purgatory came from. Did it come from the council of Trent? How do I answer my friends when I don't even know the answer? Please do not give me a "round about" answer...I need some straight, to the point answers. Thank you