In the typology of signs devised by Charles S. Peirce, the North American semiotician, the index is a sign whih has an "existential relationship" to the object it represents (i.e. the index is always spatially or temporally contiguous with the object).
Peirce contrasts the index 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).
This opposition between index and signifier underpins the following distinctions in Lacan's work.
The Psychoanalytic and Medical Concepts of the Symptom
Whereas in medicine, the symptom is regarded as an index of the disease, in psychoanalysis the symptom is not an index but a signifier.
Hence in psychoanalysis there is no one-to-one fixed link between pathological phenomena and the underlying structure.
Codes (Animal) and Language (Human)
Codes are composed of indices, whereas language is composed of signifiers.
This explains why codes lack the most important feature of language: its potential for ambiguity and equivocation.
The opposition between signifier and index is complicated by the existence of certain signifiers which also function as indices; these are called shifters.