binary relation

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A set of assignments from one set to itself or to another set. For example, “less than” is a binary relation from the set of real numbers to itself in which 3 is assigned to 4 (because 3 < 4), but 4 is not assigned to 3 (because 4 > 3).

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun set theory A relation, such as "is less than" or "is the daughter of", that makes statements about pairs of objects, these statements being true or false depending on the objects.