reciprocity

The characteristic of being reciprocal, e.g. of a relationship between people.

In a friendship, reciprocity occurs where the contribution of each party meets the expectations of the other party.

There might be benefit on one side, and obligation on the other; but their reciprocity of affection, their mutual exchange of small kindnesses—those strongest rivets of common attachment—were no more.

A reciprocal relationship.

A relation of mutual dependence or action or influence.

A reciprocal construction involves two noun phrases where each of the participants occupies both the role of agent and patient with respect to the other. see: Wikipedia:Reciprocal pronoun.

The mutual exchange of rights, privileges or obligations between nations. see: Wikipedia:Reciprocity (international relations).