(1) Several restricted applications of this general term are found in philosophical and linguistic studies of meaning, the former especially discussing the types of possible contrast involved in such notions as ‘signs’, ‘symbols’, ‘symptoms’ and ‘signals’. Sometimes ‘sign’ is used in an all-inclusive sense, as when semiotics is defined as ‘the science of signs’. In linguistic discussion, the most widespread sense is when linguistic expressions (words, sentences, etc.) are said to be ‘signs’ of the entities, states of affairs, etc., which they stand for (or, often, of the concepts involved).

(2) In such phrases as sign language and sign system, the term has a very restricted sense, referring to the system of manual communication used by certain groups as an alternative to oral communication. Such groups include policemen (in traffic control), drivers, monks vowed to silence, television studio directors, and so on; but the main application of the term is in relation to the deaf.