Definitions:
given, felt, or done in return

(of an agreement or obligation) bearing on or binding each of two parties equally

(of a course or bearing) differing from a given course or bearing by 180 degrees

(of a quantity or function) related to another so that their product is unity.

an expression or function so related to another that their product is unity; the quantity obtained by dividing the number one by a given quantity

a pronoun or verb expressing mutual action or relationship, e.g. each other, fight.