The term pirate radio usually refers to illegal or unregulated radio transmissions. Its etymology can be traced to both the illegal aspects of the transmission, as well as to the occasional use of sea vessels - fitting the most common perception of a pirate - as the base for the transmissions. The term is most commonly used to describe illegal broadcasting for entertainment or political purposes, but is also sometimes used for illegal two-way radio operation. Rules and regulations vary widely from country to country. In countries such as the USA and many countries in Europe, many types of radio licenses exist, and often the term pirate radio generally describes the unlicensed broadcasting of FM radio, AM radio, or shortwave signals over a significant coverage area that could be picked up by listeners.

Sometimes radio stations are deemed legal where the signal is transmitted, but illegal and considered "pirate stations" where the signals are received—especially when the signals cross a country's border. In other cases, a broadcast may be considered "pirate" due to the nature of its content, its transmission format (especially a failure to transmit a station identification according to regulations), or the transmit power (wattage) of the station, even if the transmission is not technically illegal (such as a webcast or an amateur radio transmission). Therefore pirate radio can sometimes mean different things to different people. Pirate radio stations are sometimes called bootleg stations (a term especially associated with two-way radio), clandestine stations or Free Radio stations.