I understand both the terms polymeter and polyrhythm, but when songs from bands such as Meshuggah are discussed the term 'hypermeter' keeps popping up. Can someone please explain?
Hypermeter is meter at a higher level, combining several measures into one unit. The figure below is taken from an academic analysis of Meshuggah's album "Catch 33" - link:
The cymbal plays a normal 4/4 beat (plus one 2/4 measure), but the snare drum and the bass drum superimpose an additional rhythm, which is indicated by the blue lines, and which can be analyzed as four 7/8 measures. This creates a hypermeter in which those four bars are combined into a larger unit.