According to Jacques Lacan, the unary trait is the elementary form of the signifier as pure difference that supports symbolic identification.

In the second of the three forms of identification described by Freud, the subject identifies regressively with a love object or rival by adopting a "single trait" of the other person (einziger Zug) (1921c, p. 107). Dora's cough, for example, was an imitation of her father's.

Lacan recognized this single trait as a signifier. Or more precisely, insofar as this signifier is isolated and is not part of a chain of signifiers, it is first a sign or an "insignia of the Other" (cf. Lacan, 1957-58, p. 304; 2002, p. 253). This insignia of the Other constitutes the nucleus of the ego-ideal.

In his seminar on Identification (1961-62), Lacan used Saussure's linguistics, to compare the einziger Zug with the signifier as a distinct element. Thus he translated it as...