in Law, q. d. "what for what," denotes the giving one thing of value for another: or the mutual consideration and performance of both par-ties 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 wrote for quo, i.e. 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 the gen-eral extended to all blunders or mistakes committed in medicine, either in the prescription, the preparation, or application of remedies.