Cel, in Animation lingo, is short for celluloid, which in turn is short for celluloid frame. An animation cel is what, when combined with other cels, creates a single frame of animation.
The reason several cels must be combined together is because different cels only have certain elements of the frame on them. Generally, a frame will consist of three to four basic types of cels; a background cel which is static (unchanging), a non-action element cel (things that exist around the characters but do not 'do' anything and the character cels, which are were the the actual action happens. There is also usually a clear top cel, but that doesn't actually have anything on it, so we don't count that.
The funny thing is that now-a-days, since more and more animation is strictly digital, animation studios will hire cel artists to make cels from computerized frames purly for sale to the public. They are, in a sense, fake cels because they were never actually used in any sort of animation production.
Cel is also a programming language based on Self which is, in turn, based on Smalltalk. In terms of attributes, Cel is: interpreted, pure Objects, prototype based, and reflective.