The KISS principle is a maxim often invoked when discussing design to fend off creeping featurism and control complexity of development. The usual explanation for this acronym is "Keep it Simple, Stupid", others are "Keep it Simple and Stupid" (often in artificial intelligence) or "keep it small and simple".
The acronym was originally used as a piece of advice from lawyers counseling their clients.
Possibly related to the common marketing maxim for sales presentations, "Keep It Short and Simple".
See also Occam's Razor, Simplicity.
This article (or an earlier version of it) contains material from FOLDOC, used with permission.
