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 characteristic of being reciprocal, e.g. of a relationship between people.
The mutual exchange of rights, privileges or obligations between nations. see: Wikipedia:Reciprocity (international relations).
The responses of individuals to the actions of others.
A transposition.
a musical instrument that is written in a different pitch to how it sounds, often for ease of playing multiple instruments of the same family that have different ranges.