What is Airplay?

Legal Definition
In radio broadcasting, airplay is how frequently a song is being played on radio stations. A song which is being played several times every day (spins) would have a large amount of airplay. Music which became very popular on jukeboxes, in nightclubs and at discotheques between the 1940s and 1960s would also have airplay.

For commercial broadcasting (television stations), airplay is usually the result of being placed into rotation, also called adding it to the station's playlist by the music director, possibly as the result of a Pay for Play sponsored by the record label. For student radio and other community radio or indie radio stations, it is often the selection by each disc jockey, usually at the suggestion of a music director.

Most countries have at least one radio airplay chart in existence, although larger countries such as Canada, the United States of America, the United Kingdom, Germany, Australia, Japan, and Brazil have several, to cover different genres and areas of the country.

A song which was successful in the airplay charts but weak in sales was commonly known as a "turntable hit" when radio stations played only vinyl singles. Airplay can be a crucial element in securing a singer's 'hit', and alongside social networking websites it is an effective method that artists use to make their name known.

Aaliyah's "Try Again" (2000) was the first song ever to reach number one on the Billboard Hot 100 based solely on the strength of its radio airplay.
-- Wikipedia
Legal Definition
When a record is played in radio or TV. The more it is played the more popular the record is.