The best definition I have ever seen is from

fluble.com:

9 out of 7 mathematicians define "recursion" as a process in which 9 out of 7 mathematicians define "recursion" as a process in which 9 out of 7 mathematicians define "recursion" as a process in which 9 out of 7 mathematicians...

The other -2 mathematicians say the entire thing is silly and leave the room, only to be pulled under the ground and devoured by giant beetles.