reciprocal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Done, given, felt, or owed in return.
• adjective Existing, experienced, or done on both sides.
• adjective Grammar Expressing mutual action or relationship. Used of some verbs and compound pronouns.
• adjective Mathematics Of or relating to the reciprocal of a quantity.
• adjective Physiology Of or relating to a neuromuscular phenomenon in which the excitation of one group of muscles is accompanied by the inhibition of another.
• adjective 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.
• noun Something that is reciprocal to something else.
• noun 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 .

from The Century Dictionary.

• Moving backward and forward; alternating; reciprocating.
• Mutually exchanged or exchangeable; concerning or given or owed by each (of two or more) with regard to the other or others: as, reciprocal aid; reciprocal rights, duties, or obligations; reciprocal love or admiration.
• Having an interchangeable character or relation; mutually equivalent or correspondent; concordant; agreeing.
• Synonyms Reciprocal, Mutual. There is a theoretical difference between these words, although it often is not important. That is mutual which is a common act on the part of both persons at the same time. Mutual is not properly applicable to physical acts or material things, as blows or gifts. Reciprocal means that one follows another, being caused by it, with emphasis upon that which is viewed as caused: as, reciprocal love or hate. See remarks under mutual as to the propriety of using mutual for common.
• noun That which is reciprocal to another thing.
• noun In mathematics, the quotient resulting from the division of unity by the quantity of which the quotient is said to be the reciprocal.
• In geometry, definitely dual, so that the dual of each element is fixed and constructible.
• noun In geometry, the dual.

from the GNU version of the Collaborative International Dictionary of English.

• noun That which is reciprocal to another thing.
• noun (Arith. & Alg.) The quotient arising from dividing unity by any quantity; thus 1/4 is the reciprocal of 4; 1/(a + b) is the reciprocal of a + b. The reciprocal of a fraction is the fraction inverted, or the denominator divided by the numerator.
• adjective Recurring in vicissitude; alternate.
• adjective Done by each to the other; interchanging or interchanged; given and received; due from each to each; mutual
• adjective Mutually interchangeable.
• adjective (Gram.) Reflexive; -- applied to pronouns and verbs, but sometimes limited to such pronouns as express mutual action.
• adjective (Math.) Used to denote different kinds of mutual relation; often with reference to the substitution of reciprocals for given quantities. See the Phrases below.
• adjective (Math.) one which remains unchanged in form when the reciprocal of the unknown quantity is substituted for that quantity.
• adjective (Geom.) two figures of the same kind (as triangles, parallelograms, prisms, etc.), so related that two sides of the one form the extremes of a proportion of which the means are the two corresponding sides of the other; in general, two figures so related that the first corresponds in some special way to the second, and the second corresponds in the same way to the first.
• adjective (Math.) a proportion such that, of four terms taken in order, the first has to the second the same ratio which the fourth has to the third, or the first has to the second the same ratio which the reciprocal of the third has to the reciprocal of the fourth. Thus, 2:5: :20:8 form a reciprocal proportion, because 2:5: :1/20:1/8.
• adjective (Math.) any two quantities which produce unity when multiplied together.
• adjective (Math.) the ratio between the reciprocals of two quantities; as, the reciprocal ratio of 4 to 9 is that of 1/4 to 1/9.
• adjective (Logic) those terms which have the same signification, and, consequently, are convertible, and may be used for each other.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Of a feeling, action or such: mutual, uniformly felt or done by each party towards the other or others; two-way.
• adjective Something that is contrary or opposite.
• noun arithmetic Of a number, the number obtained by dividing 1 by the given number; the result of exchanging the numerator and the denominator of a fraction.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective of or relating to the multiplicative inverse of a quantity or function
• noun hybridization involving a pair of crosses that reverse the sexes associated with each genotype
• adjective concerning each of two or more persons or things; especially given or done in return
• noun (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
• noun something (a term or expression or concept) that has a reciprocal relation to something else