Quid pro quo
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Quid pro quo ("this for that" in Latin) means an exchange of goods or services, where one transfer is contingent upon the other. English speakers often use the term to mean "a favor for a favor"; phrases with similar meaning include: "give and take", "tit for tat", and "you scratch my back, and I'll scratch yours."
Legal usage 
In legal usage, quid pro quo indicates that an item or a service has been traded in return for something of value, usually when the propriety or equity of the transaction is in question. For example, under the common law, a binding contract must involve consideration: that is, the exchange of something of value for something else of economic value. In the United States, if the exchange appears excessively one sided, courts in some jurisdictions may question whether a quid pro quo did actually exist and the contract may be void by law.
Similarly, political donors are legally entitled to support candidates that hold positions with which the donors agree, or which will benefit the donors. Such conduct becomes bribery only when there is an identifiable exchange between the contribution and official acts, previous or subsequent, and the term quid pro quo denotes such an exchange. The term may also be used to describe blackmail, where a person offers to refrain from some harmful conduct in return for valuable consideration.
Quid pro quo harassment occurs when employment or academic decisions or expectations (hiring, promotions, salary increases, shift or work assignments, performance standards, grades, access to recommendations, assistance with school work, etc.) are based on an employee or student's submission to or rejection of sexual advances, requests for sexual favors, or other behaviour of a sexual nature. These cases involve tangible actions that adversely affect either the conditions of work or academic progress.
Other meanings 
- Quid pro quo may sometimes be used to define a misunderstanding or blunder made by the substituting of one thing for another, particularly in the context of the transcribing of a text. In this alternate context, the phrase qui pro quo is more correct (see below).
Related phrases 
The phrase qui pro quo is used in languages such as Italian, Portuguese, Spanish and French, where it means a misunderstanding. In those languages, the phrase corresponding to the usage of quid pro quo in Latin is do ut des (that means, in English, "I give so that you will give").
The two expressions seem to have the same origin, though they have different meanings. The Vocabolario Treccani, under the entry “qui pro quo”, observes that the latter expression probably derives from the Latin ‘quid pro quo’ used in late medieval pharmaceutical compilations 
See also 
|Look up quid pro quo in Wiktionary, the free dictionary.|
- Merriam-Webster, the American Heritage Dictionary of the English Language (Fourth Edition)
- One such example is "section 2-302 of the Uniform Commercial Code".
- "Blunder made by using or putting one thing for another (now rare)" – Concise Oxford Dictionary, 4th edition, 1950.
- (Italian) Treccani, qui pro quo.
- Qui pro quo used to refer to a copying mistake made by a scribe, qui being the nominative case and quo the ablative case of the same personal pronoun. Further information may be found in the AWADmail Issue 49.
- Vocabolario Treccani, http://www.treccani.it/vocabolario/qui-pro-quo/