Sometimes written as IFS. Pretty much what each word means. Either a function or set of functions which may be applied iteratively to produce output which seems far more complex or intricate than the functions or input themselves. Examples include the rules for making a Sierpinski triangle, a Koch snowflake, and many L-systems. In a very odd way, you may consider the game of life to be an IFS where the rules map an infinite plane of cells to an infinite plane of cells, but this is a bit of a stretch. Iterated function systems are very much tied to ideas of fractals.