(grammar) That which makes something reciprocal.

(grammar, transitive) To make reciprocal.

(number theory) The mathematical theorem which states that, for given odd prime numbers p and q, the question of whether p is a square modulo q is equivalent to the question of whether q is a square modulo p.

A piston engine.

Any scientific principle that expresses various reciprocal relationships for the behaviour of a physical system.

(grammar) A pronoun referring to an antecedent in the plural, and expressing a mutual relation.

(biology) The phenomenon of two organisms acting in ways that mutually benefit one another.

(ethics) The golden rule used by philosophers.

plural of reciprocal altruism

plural of reciprocating engine