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Within a writing system, a sign is a basic unit. Similar terms which are more specific are character, letter or grapheme.

In Semiotics a sign is generally defined as "Something that stands for something else, to someone in some capacity" (Marcel Danesi and Paul Perron, "Analyzing Cultures")

This definition tries to establish the dual nature of a sign (as expressed by de Saussure) where the sign is divided into the tangible part (Saussure: signifier) and the conceptual part (Saussure: signified). It also posits the importance of both orientational and situational context in which a sign can mean - a sign has to mean something to someone for the notion of meaning to be relevant (if you like, an observer of the sign's meaning has to be present for the meaning to exist) and the *way* a sign means can change depending on the situation, culture and a few other variables.

De Saussure's definition of a sign differs from that proposed by C. S. Peirce. He proposed that there are three parts to a sign:

Perice's definition, while more obscure is also more precise because it allows for a greater degree if distinction between the elements that go into the creation of a sign.

Finally, signs are not limited to words but also include images, gestures, scents, tastes, textures, sounds - essentially all of the ways in which information can be expressed and transmitted by living beings.

In general linguistics Ferdinand de Saussure described a sign as a combination of a concept and a sound-image. A sound image is something mental as it is possible to talk to oneself without actually moving the lips. But normally the sound-images are used to produce an utterance.

So a sign consists of

Source: Ferdinand de Saussure, Course in General Linguistics; Part One - General principles; chapter Nature of the Linguistics Sign

The field which studies systems of signs is called semiotics.

In medicine, a sign is a objective evidence of the presence of a disease or disorder, as opposed to a symptom, which is subjective.
In computing, the digits where one bit among them tells the number is either negative or non-negative are called signed, otherwise unsigned.

In mathematics, the sign of a number refers to whether it is negative or positive.

On (the side of) roads there are traffic signs.

In Astrology sign is often used to mean the Sun sign.

See also signing