reciprocal (adj.)

"moving backward and forward, alternating; mutually exchanged or exchangeable; having an interchangeable character or relation, mutually equivalent and correspondent," 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.

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Definitions of reciprocal from WordNet
reciprocal (n.)
something (a term or expression or concept) that has an inverse relation to something else;
risk is the reciprocal of safety
reciprocal (n.)
(mathematics) one of a pair of numbers whose product is 1: the reciprocal of 2/3 is 3/2; the multiplicative inverse of 7 is 1/7;
Synonyms: multiplicative inverse
reciprocal (n.)
hybridization involving a pair of crosses that reverse the sexes associated with each genotype;
Synonyms: reciprocal cross
reciprocal (adj.)
concerning each of two or more persons or things; especially given or done in return;
reciprocal trade
reciprocal privileges at other clubs
reciprocal aid
Synonyms: mutual
reciprocal (adj.)
of or relating to the multiplicative inverse of a quantity or function;
the reciprocal ratio of a:b is b:a