The supplement: Derrida takes this term from Rousseau, who saw a supplement as "an inessential extra added to something complete in itself. Derrida argues that what is complete in itself cannot be added to, an so a supplement can only occur where there is an originary lack. In any binary set of terms, the second can be argued to exist in order to fill in an originary lack in the first. This relationship, in which one term secretly resides in another, Derrida calls invagination.

From the Derrida Glossary