From Jargon File (4.4.4, 14 Aug 2003) [jargon]:
demoeffect
n.
[demoscene]
1. What among hackers is called a display hack. Classical effects
include "plasma" (colorful mess), "keftales" (x*x+y*y and other
similar patterns, usually combined with color-cycling), realtime
fractals, realtime 3d graphics, etc. Historically, demo effects have
cheated as much as possible to gain more speed and more complexity,
using low-precision math and masses of assembler code and building
animation realtime are three common tricks, but use of special
hardware to fake effects is a Good Thing on the demoscene (though
this is becoming less common as platforms like the Amiga fade away).
2. [Finland] Opposite of dancing frog. The crash that happens when
you demonstrate a perfectly good prototype to a client. Plagues most
often CS students and small businesses, but there is a well-known case
involving Bill Gates demonstrating a brand new version of a major
operating system.