They indicate something. The index connected with its meaning (not arbitrary) but unlike the icon, it’s not the object itself. As examples, we can say that smoke indicates fire, smiles indicate happiness, fresh coffee smell in the morning indicates that someone preparing breakfast. Even medical symptoms and measuring instruments are indexes, because they indicate something.
They have no resemblance to the real object, it’s a result of a convention. A symbol can only make meaning if the person already knows that, so, this is a matter of culture and previous knowledge. We all know that a dove represents peace, but there’s no connection between the animal and peace, it’s just …
or semiology, is the study of signs, symbols, and signification. It is the study of how meaning is created, not what it is. Below are some brief definitions of semiotic terms, beginning with the smallest unit of meaning and proceeding towards the larger and more complex:
an acoustic image (the ‘signifiant’, or signifier); literal meaning ex. red apple
A concept (the ‘signifié’, or signified) abstract meaning ex. forbidden fruit and lust
a clear representation of the object itself, keeping its characteristics. There’s no distinction between the icon and the real object. Examples are: photos, drawings, imitations, onomatopoeias and others.