19. Recursive, adjective. In mathematics and computer science, 1) a definition that describes something in terms of itself; 2) a process that involves repeatedly using the same rule on a series of successive inputs, and, after each iteration, subjecting the result to the original rule. The process of calculating compound interest for a bank account, for example, relies on recursion.
The canonical illustration of a recursive formula is the one used to generate the Fibonacci series: Starting with the integers 0 and 1, produce the next number in the series by adding the previous two. From Latin re +currere + -ivus, “tending to run back.”
Certain types of mazes, if simple enough, can be escaped this way. Keep one hand on one wall; always turn left; always turn right; always choose randomly. Sooner or later, freedom will result. But some recursions—mirrors before mirrors—consign you to infinite reversals. The trick, in the end, is knowing how long to keep trying.