adjective

1. done, felt, given, etc. in return hoping for a reciprocal favor
2. present or existing on both sides; each to the other; mutual to feel a reciprocal affection
3. corresponding but reversed or inverted
4. equivalent or interchangeable; corresponding or complementary
5. Gram. expressing mutual action or relation “each other” is traditionally called a reciprocal pronoun
6. Math. of the reciprocals of quantities, or their relations

Etymology: < L reciprocus, returning, reciprocal < *reco-prokos, backwards and forwards < *recos (< re-, back + *cos < ?) + IE *proko-, ahead (> Gr proka, forthwith) < base *pro-, forward, ahead + -al

noun

1. anything that has a reciprocal action on or relation to another; complement, counterpart, equivalent, etc.
2. Math. the quantity resulting from the division of 1 by the given quantity; quantity which multiplied by the given quantity equals 1 (Ex.: the reciprocal of 7 is , of is 7)