A symbol is anything that represents something else than itself. There may or may not be any obvious mundane relation between the symbolic object and the thing it represents. Any material object can serve as a symbol, but so can quite imaterial things like smells, vizualizations, thought forms, concepts. The things symbols represent tend to be immaterial, such as concepts of thought, emotions, or abstract ideas.
Symbols are quite common in mundane use, although most people would not call them so. Trafic signs, for example, can be interpreted as simple symbols. The represent rules that don't have much to do with the pictures on the signs.
Sigils can be interpreted as personal symbols, although they are of course much more than that.