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1560s, with -al (1) + stem of Latin reciprocus "returning the same way, alternating," from pre-Latin *reco-proco-, from *recus (from re- "back;" see re-, + -cus, adjective formation) + *procus (from pro- "forward;" see pro-, + -cus. Related: Reciprocally. The noun meaning "that which is reciprocal" (to another) is from 1560s.
reciprocal re·cip·ro·cal (rĭ-sĭp'rə-kəl)
Of or relating to a neuromuscular phenomenon in which the excitation of one group of muscles is accompanied by the inhibition of another.
Of or being a pair of crosses in which the male parent in one cross is of the same genotype or phenotype as the female parent in the other cross.
The number by which a given number must be multiplied to get a result of one. The reciprocal of one-half, for example, is two.