A term coined by Julia Kristeva and enthusiastically adopted by literary critics in the late 20th century to describe how texts refer and allude to one another. Such practices as quotation, allusion, translation, and parody fall under this blanket term, and the general phenomenon of the influence of one writer on another can be talked about in these terms. The word is most popular among those discussing literature, but it can be used in talking about other media as well, for instance films.
For instance, when Tupac sings “Something wicked this way come” he is referring back to a moment in Shakespeare’s Macbeth when one of the witches says “By the pricking of my thumbs, Something wicked this way comes.” The assumption is that some of his listeners will recognize the borrowing, and that it will deepen their understanding of what he is saying there, by bringing in meaning from the Shakespeare context to his own meaning.