Icons, indices and symbols are the three types of signs defined by logician Charles Sanders Peirce. In other words, these terms provide the basis for a classification of the many ways a thing is considered to "stand for" or "represent" another thing.

Icon

An icon is a sign that physically resembles what it stands for. Pictures are icons because they look like the thing they represent. Onomatopoeic words are icons because they sound like the object or fact they signify.

Many icons, for example pictures, can be more or less understood immediately, with no specific training. As for other types of icons, such as onomatopoaia, resemblance is a vague concept and depends on culture and habits. See Noises animals make in different languages for examples.

And yes, the icons on your computer screen are icons, since they usually look like a printer or a file being opened.

Index

An index is a sign which occurrence implies the occurrence of some other event or object. The index can be the cause of the signified thing, or its consequence, or be merely correlated to it. For example, a smile on your face is an index of your state of mind. A page on E2 that takes 30 seconds to load is an index that the E2 Web servers are receiving more requests than they can handle simultaneously.

Understanding indices usually require some thinking or some previous knowledge. For example, I had to read E2 lag problems and (potential) solutions to understand what's happening on the Web server.

Symbol

Lazy definition: a symbol is a sign which is neither an icon or an index.
Real definition: a symbol is a sign which relation to the signified thing is conventional or arbitrary.

Most words are symbols, because they do not look like or sound like what they mean (if you believe Ferdinand de Saussure's motto, that linguistic signs are arbitrary). The colors in a flag are symbols for specific values (courage, honesty, democracy, union...).

Symbols need to be learned, not only because they are often arbitrary, but also because their form is very strict. If you change one single letter in a word, the meaning disappears entirely. If you change the orders of the colors in your flag, it will not be your flag any more. On the contrary, you may change details in a painting (icon) or in a smile (index) with little impact of the signified thing. (1)

Note that other linguists have given very different definitions to this word. The important point is the idea, not the word.


(1) This is a personal idea, I suppose that other people have said the same thing (or maybe I'm plain wrong).

Sources:
A good introduction: http://www.arts.uwa.edu.au/LingWWW/LIN101-2001/NOTES-101/signs.html
Another one: http://www.cs.indiana.edu/~port/teach/103/sign.symbol.html
and others, but you can search Google for the title of this node as I did.

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