rainfly
Definitions
A waterproof cover placed over the top of a tent to provide protection from rain and moisture.
A secondary, detachable fabric layer designed to keep a tent's interior dry while allowing for ventilation.
Examples
Make sure to secure the rainfly tightly so that water does not pool on top of the tent during the storm.
I forgot to pack the rainfly, so we got soaked when the unexpected downpour started in the middle of the night.