In the typology of signs devised by Charles S Perice, the North American semiotician, the index is a sign whih has an "existential relationship" to the object it represents. (The index is always spatially or temporally contiguous with the object).
Perice contrasts the idnex swith the symbol, which, like Saussure's concept of the sign, is characterized by the absence of all necessary connections between the sign and its object.
For example, smoke is an index of fire.
In Lacan's discourse, the term 'index' functions in opposition to the term signifier.
Lacan thus conceives the index as a 'natural' sign, one in which there is a fixed, bi-univocal correspondence between sign and object (unlike the signifier, which has no fixed link with any one signified).