In the discussion of Figures of Speech, symbols are like metaphors and similes, but are comparisons more generally agreed by a culture, or stated and developed by a writer. Often they become conventional. The whole world (note: a hyperbolic metonymy here) now recognizes the Red Cross as a symbol of charity, disaster relief and emergency aid - with a Christian background. The similar organization with an Islamic background is symbolized by a Red Crescent. And the Stars and Stripes is universally recognized as a symbol of the United States of America. These three are visual symbols - when we see the flags that carry them flying in front of a building, say, we know who is occupying the building. But they can also be used in words, as can many symbols. "My true love has my heart, and I have his", said the lover in a poem by Sir Philip Sidney, using a symbol older than the English language - but how horrible a picture, if interpreted literally!
More personally, one can see individual writers developing symbols for their own use in their writing. In The Waste Land (1922), T.S. Eliot developed a use of the idea of water to include the idea of spiritual, as well as vegetable, growth. In his later years, W.B. Yeats developed a fictional 'Crazy Jane' into a symbolic character who represented an unconventional vision of the truth. The writer and artist William Blake had several symbols to represent authoritarian figures.
There is a verb 'to symbolize' and an adjective symbolic.
- Note: in the study of such things as writing, typography and computers, symbol usually refers to an item of graphic design that represents something. For example, £ is a symbol that represents the pound sterling.