Applying a rule or formula to its results (again and again).
Example: start with 1 and apply "double" recursively:
1, 2, 4, 8, 16, 32, ...
(We double 1 to get 2, then take that result of 2 and apply "double" again to get 4, then take the 4 and double it to get 8, and so on.)
See: Recursion