sign

v. mark, indicate; write one's name; signal, gesture; make someone write his name; communicate through sign language

n. mark, symbol; signal, indication; hint, trace; notice, advertisement; astrological symbol; sign language, method of communication based on gestures and hand movements (commonly used by the hearing impaired)

A sign is an entity which signifies another entity. A natural sign is an entity which bears a causal relation to the signified entity, as thunder is a sign of storm. A conventional sign signifies by agreement, as a full stop signifies the end of a sentence. (Contrast a symbol which stands for another thing, as a flag may be a symbol of a nation)
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The at sign (@, read aloud in English as "at") is a typographic symbol used as an abbreviation for "at" in accounting and commercial invoices, in statements such as "7 widgets @ $2 ea. = $14". More recently, the at symbol has become ubiquitous because of its use in e-mail addresses.It is often referred to informally as the at symbol, the at sign, or just at. In other languages, the symbol may have a different name (see below). It has the official name commercial at in the ANSI/CCITT/Unicode character encoding standards. However, no formal English term has been officially assigned to this character.
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SIGN may refer to:A signA sign (display device)Plus and minus signs or Negative and non-negative numbersSign (semiotics)Servicio de Inteligencia de la Gendarmería Nacional, Argentine intelligence servicethe Scottish Intercollegiate Guidelines Network.hack//SIGN, an anime seriesSigns (film) by M. Night ShyamalanThe Sign, the top single of 1994 in the United States, recorded by Ace of BaseSigns (band), an american girl group.Sign (band), an icelandic band.
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(n.)
A character indicating the relation of quantities, or an operation performed upon them; as, the sign + (plus); the sign -- (minus); the sign of division Ö, and the like.
  

(n.)
A lettered board, or other conspicuous notice, placed upon or before a building, room, shop, or office to advertise the business there transacted, or the name of the person or firm carrying it on; a publicly displayed token or notice.
  

(n.)
A military emblem carried on a banner or a standard.
  

(n.)
A motion, an action, or a gesture by which a thought is expressed, or a command or a wish made known.
  

(n.)
A remarkable event, considered by the ancients as indicating the will of some deity; a prodigy; an omen.
  

(n.)
A word or a character regarded as the outward manifestation of thought; as, words are the sign of ideas.
  

(n.)
An event considered by the Jews as indicating the divine will, or as manifesting an interposition of the divine power for some special end; a miracle; a wonder.
  

(n.)
An objective evidence of disease; that is, one appreciable by some one other than the patient.
  

(n.)
Any character, as a flat, sharp, dot, etc.
  

(n.)
Any symbol or emblem which prefigures, typifles, or represents, an idea; a type; hence, sometimes, a picture.
  

(n.)
Hence, one of the gestures of pantomime, or of a language of a signs such as those used by the North American Indians, or those used by the deaf and dumb.
  

(n.)
Something serving to indicate the existence, or preserve the memory, of a thing; a token; a memorial; a monument.
  

(n.)
That by which anything is made known or represented; that which furnishes evidence; a mark; a token; an indication; a proof.
  

(n.)
That which, being external, stands for, or signifies, something internal or spiritual; -- a term used in the Church of England in speaking of an ordinance considered with reference to that which it represents.
  

(n.)
The twelfth part of the ecliptic or zodiac.
  

(n.)
To affix a signature to; to ratify by hand or seal; to subscribe in one's own handwriting.
  

(n.)
To assign or convey formally; -- used with away.
  

(n.)
To make a sign upon; to mark with a sign.
  

(n.)
To mark; to make distinguishable.
  

(n.)
To represent by a sign; to make known in a typical or emblematic manner, in distinction from speech; to signify.
  

(v. i.)
To be a sign or omen.
  

(v. i.)
To make a sign or signal; to communicate directions or intelligence by signs.
  

(v. i.)
To write one's name, esp. as a token of assent, responsibility, or obligation.
  

Any abnormality, such as a change in appearance, sensation, or function, observed by a physician when evaluating a patient which indicates a disease process.

a term introduced by the classic linguist Saussure. A sign is composed of a signified and a signifier.  For example, a signifier like "table" represents the concept of a table (not the object -table).