Medium-sized swift with a strongly contrasting rump patch. Dark slaty gray, with a lighter gray throat and a large white rump patch. Paler, more contrasting throat, and much lighter rump than in the similar Gray-rumped Swift; Short-tailed Swift has a bright white rump patch like Pale-rumped, but has a shorter tail, bigger primary bulge, and pale undertail coverts (evenly gray in Pale-rumped). Readily associates with other swift species in flocks.