Definitions of demoeffect  

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.