Wikipedia says: A demo (demonstration) is a non-interactive multimedia presentation made within the computer subculture known as the demoscene. Demogroups create demos to demonstrate their abilities in programming, music, drawing, and 3D modeling. The key difference between a classical animation and a demo is that the display of a demo is computed in real time, making computing power considerations the biggest challenge. Demos are mostly composed of 3D animations mixed with 2D effects and full screen effects.
That is a good definition. A « demo » is something artistic and technical that you simply watch. The same way you watch a painting or you listen to music. The main advantage of using computer is that all arts (drawing, music, programming) can be merged to create something beautiful and useless.
One of my A500. Photo taken in 2012. I’ve bought two A500 this year (My old A500 had died around 1990).