Within a writing system, a sign is a basic unit. Similar terms which are more specific are character, letter or grapheme.
Signs are the basic units of semiotics, which is the systematic study of signs.
In linguistics, Ferdinand de Saussure described a sign as a combination of a concept and a sound-image.
In medicine, a sign is 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. See also: Signedness