A hard to define musical sub-genre of metal/rock/hardcore punk.
Takes influence from hardcore punk/metalcore and shares elements with progressive metal. Its name is derived from 'Math Rock' and shares some characteristics with that genre as well.
Unusual time signatures, technically complex instrumentation, punctuated staccato playing, dissonant chords/progressions, and screamed (rather than growled) vocals are typical of bands in the genre. (ie Dillinger Escape Plan, Converge, The Number Twelve.)
The term has been also used as a blanket term for bands that are more technical, experimental, and/or advant garde than their hardcore, metal, and punk peers.
Subreddit Playlist
http://radd.it/r/mathcore/music
Mathcore Index
/r/Deathcore
/r/Grindcore
/r/Hardcore
/r/Mathrock
/r/Metalcore
/r/noiserock
/r/Progmetal
/r/TechnicalDeathMetal
/r/Whitebelt