Call sign
In broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...

 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 transmitting station. In North America they are used as names for broadcasting stations. A call sign can be formally assigned by a government agency, informally adopted by individuals or organizations, or even cryptographically
Cryptography is the practice and study of techniques for secure communication in the presence of third parties...

In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...

 to disguise a station's identity.

The use of call signs as unique identifiers dates to the landline railroad telegraph system.