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 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 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.

Call may refer to:

A sign is a representation of an object that implies a connection between itself and its object. A natural sign bears a causal relation to its object—for instance, thunder is a sign of storm. A conventional sign signifies by agreement, as a full stop signifies the end of a sentence. (This is in contrast to a symbol which stands for another thing, as a flag may be a symbol of a nation).

The way a sign signifies is called semiosis which is a topic of semiotics and philosophy of language.

How a sign is perceived depends upon what is intended or expressed in the semiotic relationship of:

Thus, for example, people may speak of the significance of events, the signification of characters, the meaning of sentences, or the import of a communication. Different ways of relating signs to their objects are called modes of signification.

Uses of conventional signs are varied. Usually the goal is to elicit a response or simply inform. That can be achieved by marking something, displaying a message (i.e. a notice), drawing attention or presenting evidence of an underlying cause (for instance, medical symptoms signify a disease), performing a bodily gesture, etc.