Chaff, originally called Window by the British and Düppel by the Second World War era German Luftwaffe (from the Berlin suburb where it was first developed), is a radar countermeasure in which aircraft or other targets spread a cloud of small, thin pieces of aluminium, metallized glass fibre or plastic, which either appears as a cluster of primary targets on radar screens or swamps the screen with multiple returns, in order to confuse and distract.

Modern armed forces use chaff (in naval applications, for instance, using short-range SRBOC rockets) to distract radar-guided missiles from their targets. Most military aircraft and warships have chaff dispensing systems for self-defense. An intercontinental ballistic missile may release in its midcourse phase several independent warheads as well as penetration aids such as decoy balloons and chaff.

Modern radar systems can distinguish chaff from target objects by measuring the Doppler shift; chaff quickly loses speed compared to an aircraft and thus shows a characteristic change in frequency that allows it to be filtered out. This has led to new techniques where the chaff is further illuminated by an additional signal from the target vehicle with the proper Doppler frequency. This is known as JAFF (jammer plus chaff) or CHILL (chaff-illuminated).