The Jargon File (version 4.4.7, 29 Dec 2003):

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.