Chaff is the seed casings and other inedible plant matter harvested with cereal grains such as wheat. The chaff must be separated from the grain before use, by such techniques as threshing and wind winnowing. The word "Chaff" is also used to refer to something worthless, such as in the expression "separating the wheat from the chaff", meaning to find things of value and separate them from things of no value.

Chaff is also an anti-radar technique in which aircraft or other targets spread a cloud of small, thin bits of aluminum or plastic, which appears as a cluster of secondary targets on radar screens. Devised for the USAF by the American scientist Fred Whipple (according to Harvard Gazette Archives), chaff was first used by bombers during World War II to confuse enemy radar. Modern armed forces use chaff (often deployed with short-ranged SRBOC rockets) to distract radar-guided missiles from their targets.