What is Oyer And Terminer?

Legal Definition
In English law, Oyer and terminer (/ɔːɪə ənd ˈtɜːrmɪnə/; a partial translation of the Anglo-French oyer et terminer which literally means "to hear and to determine") was the Law French name for one of the commissions by which a judge of assize sat. The commission was also known by the Law Latin name audiendo et terminando.

By the commission of oyer and terminer the commissioners (in practice the judges of assize, though other persons were named with them in the commission) were 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 was by means of the grand jury; after the grand jury had found the bills of indictment submitted to it, the commissioners proceeded to hear and determine by means of the petit jury. The words oyer and terminer were also used to denote the court which had jurisdiction to try offences within the limits to which the commission of oyer and terminer extended.
-- Wikipedia
Legal Definition
The name of a court authorized to hear and determine all treasons, felonies and misdemeanors; and, generally, invested with other power in relation to the punishment of offenders.
-- Bouviers Law Dictionary
Legal Definition
A half French phrase applied in England to the assizes, which are so called from the commission of oyer and terminer directed to the judges, empowering them to "inquire, hear, and determine" all treasons, felonies, and misdemeanors. This commission is now issued regularly, but was formerly used only on particular occasions, as upon sudden outrage or insurrection in any place. In the United States, the higher criminal courts are called "courts of oyer and terminer." Bur-rill.
-- Black's Law Dictionary
Legal Definition
To hear and determine. See Courts of oyer and terminer.
-- Ballentine's Law Dictionary