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.
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A reciprocal relationship.
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A relation of mutual dependence or action or influence.
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(grammar) 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.