**QUID pro quo**, in Law, denotes the giving one thing of value for another; or the mutual consideration and performance of both parties to a contract.

**Quid pro Quo, or Qui pro Quo**, is also used in physic to express a mistake in the physician's bill, where *quid* is written for *quo*, that is, one thing for another; or of the apothecary in reading *quid* for *quo*, and giving the patient the wrong medicine. Hence the term is in general extended to all blunders or mistakes committed in medicine, either in the prescription, the preparation, or the application of remedies.