OYER AND TERMINER, in English law, is one of the commissions by which a judge of assize sits (see ASSIZE). By the commission of oyer and terminer the commissioners (in practice the judges of assize, though other persons are named with them in the commission) are commanded to make diligent inquiry into all treasons, felonies, and misdemeanours whatever committed in the counties specified in the commission, and to hear and determine the same according to law. The inquiry is by means of the grand jury ; after the grand jury has found the bills submitted to it, the commissioners proceed to hear and determine (oyer and terminer) by means of the petty jury. The words oyer and terminer are also used to denote the court which has jurisdiction to try offences within the limits to which the commission of oyer and terminer extends.

By 7 Anne c. 21 the crown has power to issue commissions of oyer and terminer in Scotland for the trial of treason and mis-prision of treason. Three of the lords of justiciary must be in any such commission. An indictment for either of the offences mentioned may be removed by certiorari from the court of oyer and terminer into the court of justiciary.
In the United States oyer and terminer is the name given to courts of criminal jurisdiction in some States, e.g., New York, New Jersey, Pennsylvania, and Georgia.