In jazz the term means using note values twice as fast as previously but without changing the pace of the chord progressions. It is often used during improvised solos.
"Double time [is] doubling a rhythm pattern within its original bar structure.":
1 2 3 4
1 2 3 4 1 2 3 4
Rhythm pattern characteristic of much popular music including rock, quarter-note (crotchet) or "regular" time play (help·info)
Double-time: notice the snare moves to the "&" beats while the hi-hat begins to subdivide sixteenth notes (semiquavers).Play (help·info) Note also, for example, that the eighth notes (quavers) 'sound like' quarter notes (crotchets) in two tiny measures (bars).