A sign is a token or indication of something. It may be an action, event, object or pattern that conveys a meaning or a symbolic notice containing an advertisement, bearing, direction or warning that is in public view; e.g. road signs.
Sign may also refer to the evidence of existence; as in "he showed signs of life" or "there was no sign of them anywhere". Smoke is a reliable sign of the hidden presence of burning heat or fire. Catholics, for example, see the consecrated forms of the bread and wine as actual signs of the real presence of Jesus Christ himself, not simply as being only representative symbols.
In mathematics, signs can refer to the symbols of addition, subtraction, multiplication and division as well as some more advanced operations. The sign of a number refers to whether it is positive or negative.
A sign may be a meaningful gesture as in sign language or to indicate approval (a "thumbs-up sign"), disapproval or some other emotion.