quid pro quo

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Something that is given in return for something else or accepted as a reciprocal part of an exchange.

from The Century Dictionary.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Something understood as another ; an equivocation.
• noun law This for that; giving something to receive something else ; something equivalent ; something in return.
• noun An equal exchange.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun something for something; that which a party receives (or is promised) in return for something he does or gives or promises