Done, given, felt, or owed in return: a reciprocal invitation to lunch. Existing, experienced, or done on both sides: reciprocal agreements between nations; reciprocal admiration between friends. Grammar Expressing mutual action or relationship. Used of some verbs and compound pronouns. Mathematics Of or relating to the reciprocal of a quantity. Physiology Of or relating to a neuromuscular phenomenon in which the excitation of one group of muscles is accompanied by the inhibition of another. Genetics Of or designating a pair of crosses in which the male or female parent in one cross is of the same genotype or phenotype as the complementary female or male parent in the other cross. Something that is reciprocal to something else. Mathematics A number related to another in such a way that when multiplied together their product is 1. For example, the reciprocal of 7 is 1/7 ; the reciprocal of 2/3 is 3/2 .