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Longman Dictionary of Contemporary English
call sign
noun
EXAMPLES FROM CORPUS
▪ And you know Omega's call sign, don't you?
▪ He tapped out the call sign in two-minute segments, waited for ten, called again.
▪ Her call sign was Gaslight Lil and his was Popeye.
▪ I was developing the habit of calling everything by its call sign.
▪ The number six in the call sign always meant boss, chief shit.
Wiktionary
call sign

alt. 1 (context broadcasting English) A combination of letters and numbers used to identify a radio or television station. 2 (context military English) Any combination of characters or pronounceable words, which identifies a communication facility, a command, an authority, an activity, or a unit; used primarily for establishing and maintaining communications. Also called CS. (JP 1-02 Department of Defense Dictionary of Military and Associated Terms.) n. 1 (context broadcasting English) A combination of letters and numbers used to identify a radio or television station. 2 (context military English) Any combination of characters or pronounceable words, which identifies a communication facility, a command, an authority, an activity, or a unit; used primarily for establishing and maintaining communications. Also called CS. (JP 1-02 Department of Defense Dictionary of Military and Associated Terms.)

Wikipedia
Call sign

In broadcasting and radio communications, a call sign (also known as a call name or call letters—and historically as a call signal—or abbreviated as a call) is a unique designation for a transmitter station. In North America, they are used for all FCC licensed transmitters. A call sign can be formally assigned by a government agency, informally adopted by individuals or organizations, or even cryptographically encoded to disguise a station's identity.

The use of call signs as unique identifiers dates to the landline railroad telegraph system. Because there was only one telegraph line linking all railroad stations, there needed to be a way to address each one when sending a telegram. In order to save time, two-letter identifiers were adopted for this purpose. This pattern continued in radiotelegraph operation; radio companies initially assigned two-letter identifiers to coastal stations and stations aboard ships at sea. These were not globally unique, so a one-letter company identifier (for instance, 'M' and two letters as a Marconi station) was later added. By 1912, the need to quickly identify stations operated by multiple companies in multiple nations required an international standard; an ITU prefix would be used to identify a country, and the rest of the call sign an individual station in that country.

Usage examples of "call sign".

She flicked her eyes up to it, bringing it into focus and smiled as she recognised the call sign.

The Navy is on channel four, call sign Hawk-Three, standing by for ground support if you need it.

Because she was a Jedi and carried a lightsaber, and because she used a flight stick to control the X-wing, her compatriots had given her the call sign Sticks.