Something that on the surface is its literal self but which also has another meaning or even several meanings. For example, a sword may be a sword and also symbolize justice. A symbol may be said to embody an idea. There are two general types of symbols: universal symbols that embody universally recognizable meanings wherever used, such as light to symbolize knowledge, a skull to symbolize death, etc., and constructed symbols that are given symbolic meaning by the way an author uses them in a literary work, as the white whale becomes a symbol of evil in Moby Dick.
Harris, Robert. "A Glossary of Literary Terms." Virtual Salt. 4 Jan. 2002.
Accessed: 10 Nov. 2005. Avail. Online: <http://www.virtualsalt.com/litterms.htm>
Mr. K's note: Harris' definition seems to encompass the notion of an archetype, in that he discusses universal "recognizable meanings wherever used." Myths, dreams, and symbols are interconnected in that a symbol can represent a larger structure, certain components in our dreams are deeply symbolic and may represent archetypal patterns, and dreams and symbols can uniquely align themselves with the myths of our cultural reality.