Double time, in music, means that you play at twice the original
tempo. Of course, this can be done with any type of music, but it is
common only in
jazz. Sometimes, to
spice up a solo during a mid-tempo tune, the band can go into double time. The chords then change at double the speed, and everyone plays two times as fast. The band will usually return to the
original tempo at the end of the solo. Because the tempo is
exactly doubled, this sounds natural to the ear.
The change between tempos is almost always done (I'd wager 99.9' percent of the time) at the start of a chorus.
Double time feel is similar to double time. Everyone plays twice as fast, but the chords change at the original tempo. It is actually double time, with the chords stretched out on two bars each, instead of one.
See also half time, half time feel.