binary relation
Additive relation, a many-valued homomorphism between modules
Allegory (category theory)
Category of relations, a category having sets as objects and binary relations as morphisms
Confluence (term rewriting), discusses several unusual but fundamental properties of binary relations
Correspondence (algebraic geometry), a binary relation defined by algebraic equations
Hasse diagram, a graphic means to display an order relation
Incidence structure, a heterogeneous relation between set of points and lines
Logic of relatives, a theory of relations by Charles Sanders Peirce
Order theory, investigates properties of order relations