late 14c., "judicial investigation, act or process of inquiring," from Old French inquisicion "inquiry, investigation" (12c.), from Latin inquisitionem (nominative inquisitio) "a searching into, legal examination," noun of action from past participle stem of inquirere (see inquire).
In Church history, inquisitors were appointed from 382 C.E. to root out heretics, and the Inquisition refers to the ecclesiastical court (Congregation of the Holy Office) appointed 13c. by Innocent III to suppress heresy. It never operated in Britain. The capital letter form appeared in English only after c.1500, and usually refers to the office's reorganization 1478-1483 in Spain as what is commonly called the Spanish Inquisition.