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Definitions of sign:

noun:   a perceptible indication of something not immediately apparent (as a visible clue that something has happened)
Example: "He showed signs of strain"

noun:   a public display of a (usually written) message
Example: "He posted signs in all the shop windows"

noun:   a character indicating a relation between quantities
Example: "Don't forget the minus sign"

noun:   a gesture that is part of a sign language
noun:   a fundamental linguistic unit linking a signifier to that which is signified
Example: "The bond between the signifier and the signified is arbitrary--de Saussure"

noun:   (medicine) any objective evidence of the presence of a disorder or disease
Example: "There were no signs of asphixiation"

noun:   structure displaying a board on which advertisements can be posted
Example: "The highway was lined with signboards"

noun:   an event that is experienced as indicating important things to come
Example: "It was a sign from God"

noun:   having an indicated pole (as the distinction between positive and negative electric charges)
Example: "Charges of opposite sign"

noun:   any communication that encodes a message
Example: "Signals from the boat suddenly stopped"

noun:   (astrology) one of 12 equal areas into which the zodiac is divided
verb:   engage by written agreement
Example: "They signed two new pitchers for the next season"

verb:   approve and express assent, responsibility, or obligation
Example: "Have you signed your contract yet?"

verb:   be engaged by a written agreement
Example: "He signed to play the casino on Dec. 18"

verb:   communicate in sign language
Example: "I don't know how to sign, so I could not communicate with my deaf cousin"

verb:   mark with one's signature; write one's name (on)
Example: "She signed the letter and sent it off"

verb:   communicate silently and non-verbally by signals or signs
Example: "He signed his disapproval with a dismissive hand gesture"

verb:   place signs, as along a road
Example: "Sign an intersection"

verb:   make the sign of the cross over someone in order to call on God for protection; consecrate

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Alternative Terms
gesture badge evidence index indication indicator manifestation mark note signification stamp symptom token witness show-hide expression gesticulation motion signaller high-sign express-similar-words bill notice placard poster character symbol augury forerunner foretoken omen portent prefigurement presage prognostic foresight-similar-words warn-invite autograph endorse inscribe subscribe undersign law-similar-words gesticulate signalize enlist enroll enter join participate-abstain cede deed grant alien assign convey transfer give-take-reciprocity signed-sealed-and-delivered sign-in sign-on-the-dotted-line sign-out sign-over sign-up show-signs-of zodiac signifier signified arbitrary referent icon semiotics paranormal portend ecstatic mundane hallucination julian unnatural broadside sign-semiotics biohazard-disambiguation cigarette philosophy-of-language meaning-linguistics meaning-semiotics epistemology aristotle st-augustine-of-hippo thomas-aquinas language discourse communication philosophy science logic mathematics poetry theology religion