reciprocal

adjective

• 1given, felt, or done in return:she was hoping for some reciprocal comment or gesture

• 2(of an agreement or obligation) bearing on or binding each of two parties equally:the treaty is a bilateral commitment with reciprocal rights and duties
• Grammar (of a pronoun or verb) expressing mutual action or relationship.

• 3(of a course or bearing) differing from a given course or bearing by 180 degrees: he took up a reciprocal heading and dropped down to 2,000 ft

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

noun

• 1 Mathematics 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: the compressibility is the reciprocal of the bulk modulus

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