In general linguistics Ferdinand de Saussure described a sign as a combination of a concept and a sound-image. A sound image is something mental as it is possible to talk to oneself without actually moving the lips. But normally the sound-images are used to produce an utterance.

So a sign consists of:

• a concept - respectively the signified (signifié)
• a sound-image - respectively the signifier (signifiant), or phonological form in generative terms..

References

• Saussure, Ferdinand de (1916). Nature of the Linguistics Sign. In Charles Bally & Albert Sechehaye (Ed.), Cours de linguistique générale. McGraw Hill Education. ISBN 0070165246.