Why is the ninth month called September?
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.
A court established by the Roman Catholic Church in the thirteenth century to try cases of heresy and other offenses against the church. Those convicted could be handed over to the civil authorities for punishment, including execution.
Note: The Inquisition was most active in Spain, especially under Tomás de Torquemada; its officials sometimes gained confessions through torture. It did not cease operation in the Spanish Empire until the nineteenth century.
Note: By association, a harsh or unjust trial or interrogation may be called an “inquisition.”