A now famous term used by T.S.Eliot in an essay "on Hamlet" (1919). The relevant passage is: "The only way of expressing emotion in the form of art is by finding an "objective correlative"; in other words, a set of objects, a situation, achain of event which shall be the formula of the particular emotion; such that when the externalfacts, which must terminate in sensory experience, are given, the emotion is immediately evoked.
Mr Eliot explains that a work of art can be valued if it is put into relation with the works of the tradition.
The foundamental concept is the objective correlative (objective: a concrete reference; correlative: something that puts into relation something else). Eliot wants to know what is the most effective and realist method to communicate emotions (something that isn't concrete).
According to Mr Eliot, Shakespeare succeded in the creation of Lady Macbeth's soliloquy because he was able to find the objective correlative to express emotions. In Hamlet tradegy, instead, Shakespeare did't menage to render Hamlet's madness because he adopted an inadeguate use of the words, that is there is an excess of words and, as a consequence, there is not balance and order. In order to produce a work of art and express emotions, the form, that is the words, and the material, that is the reality the words appeal to, have to form a unique entity. Furthermore the unity implies a work of art is detatched, impersonal and universal.