You already know what a high score is.
Speedrunwiki.com is not exclusively devoted to speed runs. Unlike the Speed Demos Archive, with which we are not affiliated, this site is actually devoted to all forms of intense, advanced, difficult videogaming. This happily embraces the theory and practice of getting impossibly high scores in what are essentially fixed-length games such as Ikaruga and... and... the letter-sorting sub-game in The Legend of Zelda: The Wind Waker, and open-ended games like Tetris.
Yes, this does mean the site is slightly mis-named. Watch carefully as we shrug. *shrug*
Extreme high scores
The highest known high score of any kind at all is a score of 1,799,746,631,743,346,000 points obtained by Stuart Campbell on stage 3 of Score Attack mode in Giga Wing 2 for the Dreamcast.