Etymon. Yet another word I didn't know. I suppose I could think of it as the ancestor of a derivative.
My high school Latin teacher assigned us the project of compiling a derivative notebook, where we had to find words in newspapers or magazines, and look up the Latin words they were derived from. I think that may be almost the only high school assignment I remember, and it's probably where I learned the pleasure of and curiosity about words. Thank you, Miss Bortz!