What is Nisi Prius?

Legal Definition
Nisi prius (/ˈns ˈprəs/) is a historical term in English law. In the nineteenth century, it came to be used to denote generally all legal actions tried before judges of the King's Bench Division and in the early twentieth century for actions tried at assize by a judge given a commission. Used in that way, the term has had no currency since the abolition of assizes in 1971.
-- Wikipedia
Legal Definition
These words, which signify 'unless before,' are the name of a court. The name originated as follows: Formerly, an action was triable only in the court where it was brought. But, it was provided by Magna Charta, in ease of the subject, that assises of novel disseisin and mort d'ancestor (then the most usual remedies,) should thenceforward instead of being tried at Westminster, in the superior court, be taken in their proper counties; and for this purpose justices were to be sent into every county once a year, to take these assises there. 1 Reeves, 246; 2 Inst. 422, 3, 4. These local trials being found convenient, were applied not only to assises, but to other actions; for, by the statute of 13 Edw. I. c. 30, it is provided as the general course of proceeding, that writs of venire for summoning juries in the superior courts, shall be in the following form. Praecipimus tibi quod veneri facias coram justiciariis nostris apud Westm. in Octabis Seti Michaelis, nisi talis et talis tali, die et loco ad partes illas venerint, duodecim, &c. Thus the trial was to be had at Westminster, only in the event of its not previously taking place in the county, before the justices appointed to take the assises. It is this provision of the statute of Nisi Prius, enforeed by the subsequent statute of 14 Ed. III. c. 16, which authorizes, in England, a trial before the justices of assises, in lieu of the superior court, and gives it the name of a trial by nisi prius. Steph. Pl. App. xxxiv.; 3 Bl. Com. 58; 1 Reeves, 245, 382; 2 Reeves, 170; 2 Com. Dig. Courts, D b, page 316.

2. Where courts bearing this name exist in the United States, they are instituted by statutory provision. 4 W. & S. 404.
-- Bouviers Law Dictionary
Legal Definition
The nisi prius courts are such as are held for the trial of issues of fact before a jury and one presiding judge. In America the phrase is familiarly used to denote the forum (whatever may be its statutory name) in which the cause was tried to a jury, as distinguished from the appellate court. See 3 BI. Comm. 58.
-- Black's Law Dictionary
Legal Definition
Unless before; a jury trial of issues of fact before a single judge; the court held by a judge on his circuit. See 3 BL Comm. 58.
-- Ballentine's Law Dictionary