flysheet

nounC1Very rare — outside the 30,000 commonest English words, so you will meet it in writing rather than in speechToo rare in one of the two pools to compare — fewer than 20 uses in speech or in writing

A flysheet is the outer waterproof skin over a tent. Pitch the flysheet first, a flysheet in the wind, the flysheet leaked.

The gap between it and the inner is what keeps you dry, since condensation forms on the flysheet rather than on you, and touching it from inside makes water come through. Some tents pitch flysheet first, which is what you want in the rain.