quid pro quo

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

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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