The word assize refers to the sittings or sessions (Old French assises) of the judges, known as "justices of assize", who were judges who travelled across the seven circuits of England and Wales on commissions of "oyer and terminer", setting up court and summoning juries at the various assize towns.
Assize < Middle English assise < Old French assize ("session, legal action" – past participle of asseoir, "to seat") < Vulgar Latin *assedēre < Latin assidēre ("to sit beside, assist in the office of a judge") < ad + sedēre ("to sit").
Justices of the Court of King's Bench, justices of the Court of Common Pleas, and barons of the Exchequer of Pleas travelled around the country on five commissions, upon which their jurisdiction depended. Their civil commissions were the commission of assize and the commission of nisi prius. Their criminal commissions were the commission of the peace, the commission of oyer and terminer and the commission of gaol delivery.
By the Assize of Clarendon 1166, King Henry II established trial by jury by a grand assize of twelve knights in land disputes, and provided for itinerant justices to set up county courts. Prior to the enactment of Magna Carta 1215, writs of assize had to be tried at Westminster or await trial at the septennial circuit of justices in eyre, but the great charter provided that land disputes should be tried by annual assizes.
An Act passed in the reign of King Edward I provided that writs summoning juries to Westminster were to appoint a time and place for hearing the causes with the county of origin. Thus they were known as writs of nisi prius (Latin "unless before"): the jury would hear the case at Westminster unless the king's justices had assembled a court in the county to deal with the case beforehand. The commission of oyer and terminer, was a general commission to hear and decide cases, while the commission of gaol delivery required the justices to try all prisoners held in the gaols.