quid pro quo

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An equal exchange or substitution.

Century Dictionary and Cyclopedia

1. n. Something given for something else; a tit for tat; in law, an equivalent; a thing given or offered in exchange for or in consideration of another; the mutual consideration and performance of either party as toward the other in a contract.

Wiktionary

1. n. Something understood as another ; an equivocation.
2. n. law This for that; giving something to receive something else ; something equivalent ; something in return.
3. n. An equal exchange.

WordNet 3.0

1. n. something for something; that which a party receives (or is promised) in return for something he does or gives or promises