reciprocal

adj

1. (not-comparable)Of a feeling, action or such: mutual, uniformly felt or done by each party towards the other or others; two-way.

   “reciprocal love”

   “reciprocal duties”

   “Let our reciprocall vowes be remembred.”

2. (not-comparable)Mutually interchangeable.

   “These two rules will render a definition reciprocal with the thing defined.”

3. (not-comparable)Expressing mutual action, applied to pronouns and verbs; also in a broad sense: reflexive.
4. (not-comparable)Used to denote different kinds of mutual relation; often with reference to the substitution of reciprocals for given quantities.
5. (not-comparable)Done, given, felt, or owed in return.

   “a reciprocal invitation to lunch”

noun

1. The number obtained by dividing 1 by another given number; the result of exchanging the numerator and the denominator of a fraction.

   “0.5 is the reciprocal of 2.”

2. A construction expressing mutual action.

   “Depending on where reciprocalization applies (syntax vs. lexicon), the relevant reciprocal verbs are claimed to exhibit specific properties, in particular: (i) syntactic reciprocals are fully productive whereas lexical reciprocals have only limited productivity; […]”