quid pro quo

1560s, from Latin, literally "something for something, one thing for another," from nominative and ablative neuter singulars of relative pronoun qui "who" (from PIE root *kwo-, stem of relative and interrogative pronouns) + pro "for" (see pro-) + quo, ablative of quid.

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Definitions of quid pro quo from WordNet

quid pro quo (n.)
something for something; that which a party receives (or is promised) in return for something he does or gives or promises;
Synonyms: quid