1. Something visible that by association or convention represents something else which is invisible.
2. A type of defense mechanism in which one idea or object comes to represent another because of the similarity or an association between them.
3. A general mechanism in all human thinking by which some mental representation comes to stand for some other thing, class of things, or an attribute of something.

This mechanism underlies dream formation and some symptoms; such as, conversion reactions, obsessions, and compulsions.

4. In psychoanalysis, an unconscious mental process during which one object, or idea, stands for something else through some part, quality, or aspect that the two share in common, with the symbol carrying the emotional feelings which are in the initial object or idea.