A = A: what is the term for a phrase which defines itself?

Thanks to all -- I of course finally remmebered what I was looking for on my own. The best answer is chosen because it is 1) supported by citation, 2) best fits the request that it be used as a term of English/Rhetoric, and 3) was what I was looking for. The other answers are good too, and it was probabaly the unclear question which lead to those answers instead of the one I was looking for.

Thanks all.