In semiotics the meaning of the term icon is limited to a representation that resembles its object. The other two phenomenological categories of signs are index and symbol.
The index carries an actual connection to its object, such as a sign of a telephone on the door of a phone booth. A symbol in contrast has no visual connection to its object. It is an abstract sign that can only be understood through learning its meaning (Rayan & Hubner, 2006). However in the context of interface design the icon has acquired a more general meaning where it stands for any visual representation that denotes a function or content type. Compared to semiotics it covers all the three major phenomenological categories.
Rayan, A., Hubner, R. (2006) Pictograms, Icons and Signs, Thames & Hudson Ltd., London.