After giving a lecture on depth (noncommutative ring theory) in Jena, a chinese fellow raised his hand and asked what all this had to do with depth (ring theory) he had seen on the wiki. After a laugh (who owns the concept ring theory? commutative ring theorists? see the definition of ring on wiki for one possibility) and everyone else ducked the work, I became "it", to be the one who would write something. I am of course doing the best I can as an employed mathematician to write something encyclopedic (and stay away from territorial urinations) as well as elementary enough for the swimming dogs out there.