A term introduced by T.S Eliot in his essay “Hamlet and His Problems” (1919). Eliot observes that there is something in Hamlet which Shakespeare cannot “drag into the light, contemplate, or manipulate into art” , at least not in the same way that he can with Othello's jealousy, or Coriolanus' pride. He goes on to deduce that “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, a chain of events which shall be the formula for that particular emotion; such that when the external facts, which must terminate in a sensory experience, are given, the emotion is immediately evoked.”
To simplify his words, if writers or poets or playwrights want to create an emotional reaction in the audience, they must find a combination of images, objects, or description evoking the appropriate emotion. The source of the emotional reaction isn't in one particular object, one particular image, or one particular word. Instead, the emotion originates in the combination of these phenomena when they appear together.