Applying a rule or formula to its own result, 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.)