"Math Rock" is a term used to describe a kind of indie music which emerged in the late 80's; I'm not sure who coined the term. Math Rock can be described (as one reviewer put it) "prog rock with overdue student loan
payments". The music is deliberately complicated, and usually fuses together elements that normally would not work together... guitarists usually work on
small "moments" of sound rather than focus on solos; bass work is sometimes influenced by dub, and the drums are usually syncopated. The singers in Math
Rock bands usually whisper their vocals. Some bands use extreme shifts in volume (though oddly, almost never tempo) to create emotional force in the
music. Math Rock bands also tend to be short lived, and the "scene" has consisted (for a number of years) of musicians who have played in each other's bands at various points. Others have gone on to form a few modern chamber music bands. The general consensus is that math rock's defining album is Slint's Spiderland. Some other bands: Rex, June of '44 ("Four Great Points" is my favorite math rock record), the For Carnation. Some of the "chamber music" bands: Rachel's, Godspeed you Black Emperor!, Windsor for the Derby.
It is worth noting that, with few exceptions, it hasn't had much of an impact outside of itself.
This was previously submitted to the Silence list, and has been posted on http://www.newalbion.com/artists/cagej/silence/html/2000q3/0071.html.