The at sign or @ is also commonly called the at symbol or commercial at in English — and much less commonly a wide range of other terms. The fact that there is no single word in English for the symbol has prompted some writers to use the French arobase or Spanish arroba — or to coin new words such as apserand and ampersat - but none of these has achieved wide currency.

Originally an accounting and commercial invoice abbreviation meaning "at the rate of" (e.g. 7 widgets @ $2 = $14), it was not included on the keyboard of the earliest commercially successful typewriters, but was on at least one 1889 model and the very successful Underwood models from the "Underwood No. 5" in 1900 onward. It is now universally included on computer keyboards.

In recent years, its meaning has grown to include the sense of being "located at" or "directed at", especially in email addresses and social media like Facebook and Twitter.

The mark is encoded at U+0040 @ commercial at (HTML: @).

There are several theories about the origin of the commercial at character.

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.