A memo function remembers which arguments it has been called with and the result returned and, if called with the same arguments again, it will simply return the result from its memory rather than recalculating it.
This same principle is found at the hardware level in computer architectures which use a cache to store recently accessed memory locations.
A Common Lisp package by Marty Hall <hall@aplcenmp.apl.jhu.edu> which supports memoisation is available by FTP.
(02 Nov 1994)