1. An action or an arrangement that involves two people or groups of people who behave in the same way or agree to help each other and give each other advantages.
2. A returning, usually for something given.
3. A mutual giving and receiving.
4. In prosthodontics, the means by which one part of an appliance is made to counter the effect created by another part.

Prosthodontics refers to a dental specialty concerned with the restoration and maintenance of oral function by the replacement of missing teeth and structures by artificial devices or prostheses.

5. In electronics, a process by which a reciprocal impedance (or network) is derived from a given impedance (or network).