CQ is a code used by wireless operators, particularly those communicating in Morse code, (-.-. --.-), but also by voice operators, to make a general call (called a CQ call). Transmitting the letters CQ on a particular radio frequency is an invitation for any operators listening on that frequency to respond. It is still widely used in amateur radio.
The CQ call was originally used by landline telegraphy operators in the United Kingdom. French was, and still is, the official language for international postal services, and the word, sécurité, was used to mean "safety" or "pay attention". It is still used in this sense in international telecommunications. The letters CQ, when pronounced in French, resemble the first two syllables of sécurité, and were therefore used as shorthand for the word. In English-speaking countries, the origin of the abbreviation was popularly changed to the phrase "seek you" or, later, when used in the CQD distress call, "Calling all distress".
CQ was adopted by the Marconi Company in 1904 for use in wireless (spark) telegraphy, and was adopted internationally at the 1912 London International Radiotelegraph Convention, and is still used.
A variant of the CQ call, CQD, was the first code used as a distress signal. It was proposed by the Marconi Company and adopted in 1904, but was replaced between 1906 and 1908 by the SOS code. When the Titanic sank in 1912, however, it initially transmitted the distress call CQD DE MGY, MGY being the ship's call sign. (The Titanic's radio operator alternated between SOS and CQD afterward.)
In ham radio usage, a CQ call can be qualified by appending more letters, as in CQ DX (meaning "calling all long-distance stations"), or the ITU call sign prefix for a particular country (e.g. CQ VK for "calling Australia"). The originator of the call can be identified by appending the letters DE (French for "from", also means "this is...") and the call sign of the transmitting station. CQ is not used on VHF or UHF repeaters.
The code was used as an intro to the song Communications by Slim Gaillard.