The term pirate raio 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 short wave signals over a significant coverage area that could be picked up by listeners.