It doesn't have anything to do with 'how frequently' something is broadcast as stated in the definition below. A song could be broadcast once and can still be said to have received airplay.
Airplay is a technical term used in the radio industry to state how frequently a song is being played on a radio station. For example, a song which is being played several times every day would be classed as receiving a large amount of airplay. The term is also used in the same way regarding music video channels, to state how often a music video is being played.
Most countries have at least one radio airplay chart in existence, although larger countries such as the United States of America, the United Kingdom and Japan have several, to cover different genres and areas of the country.
A song which was very successful in the airplay charts but not so strong in sales was commonly known as "turntable hit" when radio stations only played vinyl singles.
Airplay can be crucial in securing a singers "Hit".