The signifier (signifiant) is one half of Saussure’s dyadic model of the sign. It is the form of the sign — the sound-image, the written mark, the gesture — as distinct from the concept it evokes (the signified).

Saussure insisted that the signifier is not the physical sound itself but the mental impression of it — what he called the “sound-image.” The relationship between signifier and signified is arbitrary: there is no natural connection between the sound /tri:/ and the concept of a tree. The pairing is conventional and differs across languages (de Saussure, 1916).

In Peirce’s triadic model, the roughly corresponding concept is the representamen — the sign vehicle. But the correspondence is imperfect: Peirce’s representamen participates in a three-place relation (with object and interpretant), while Saussure’s signifier participates in a two-place relation (with signified only).

Source: Saussure, Ferdinand de. Course in General Linguistics. Translated by Wade Baskin. New York: McGraw-Hill, 1959 [1916].

de Saussure, F. (1916). Course in General Linguistics. McGraw-Hill.