Usually unofficial term for an anti-publicity stunt performed by radio stations. When a station is changing format (say, from an R&B format to techno), management needs to drive away listeners in a hurry so they can build a new listener base that their advertisers can build demographics on. Usually this is accomplished by broadcasting horrid noise, sonic shrapnel, or usually just boring audio for around 48 hours.
Some more notable stunts have been a computer generated voice countdown for two days, playing the same song over and over, or just a looped recording of someone screaming. I remember driving through the desert one summer and hearing several hours of background noise from Star Trek's bridge set.
The most recent, and highest publicized, listener wash was Quick 96.