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reciprocal (adj.)

1570s, "existing on both sides, exclusive or interchangeable" (of duties, etc.), 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.

From 1590s as "given, felt, or shown in return;" c. 1600 as "corresponding or answering to each other, mutually equivalent." The sense of "moving backward and forward, having an alternating back and forth motion" (c. 1600) is obsolete. The noun meaning "that which is reciprocal" (to another) is from 1560s. In scientific and mechanical uses, reciprocating, reciprocative (1804), and reciprocatory (1826) have been tried.

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Definitions of reciprocal from WordNet
1
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
2
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
From wordnet.princeton.edu