A sign is a token or indication of something. It may be an action, event, object or pattern that conveys a meaning or a notice containing an advertisement, bearing, direction or warning that is in public view; e.g. road signs.
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.
In Christianity, a sign is a disclosure of underlying reality.