reciprocal

Short definition from Wiktionary

Adjective

1. 1
   Of a feeling, action or such: mutual, uniformly felt or done by each party towards the other or others; two-way.
2. 2
   Mutually interchangeable.
3. 3
   Expressing mutual action, applied to pronouns and verbs; also in a broad sense: reflexive.
4. 4
   Used to denote different kinds of mutual relation; often with reference to the substitution of reciprocals for given quantities.
5. 5
   Done, given, felt, or owed in return.

Noun

1. 1
   The number obtained by dividing 1 by another given number; the result of exchanging the numerator and the denominator of a fraction.
2. 2
   A construction expressing mutual action.