or relationship, n. 1. an association between, or a condition satisfied by, ordered pairs of objects, numbers, etc., such as ab = 1,... is greater than... or... is the father of .... 2. formally, any set of ordered pairs. Such a set defines a relation between the first member of each pair and its corresponding second member. If each first member is associated with only one second member, the relation is a function. See also correspondence, equivalence relation, set-valued function. 3. any analogous association of three or more members; a set of ordered n-tuples; an n-place predicate. 4. (Group theory) one of a number of conditions subject to which a set generates a presentation of a given group.