(adj)
concerning each of two or more persons or things; especially given or done in return; "reciprocal aid"; "reciprocal trade"; "mutual respect"; "reciprocal privileges at other clubs"

of or relating to the multiplicative inverse of a quantity or function; "the reciprocal ratio of a:b is b:a"

(noun)
something (a term or expression or concept) that has an inverse relation to something else; "risk is the reciprocal of safety"

(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