The Collaborative International Dictionary
Nisi \Ni"si\, conj. [L.] Unless; if not; -- used mostly in law.
Note: In legal proceedings, this word is used to indicate that any order, etc., shall take effect at a given time, unless before that time the order, etc., in modified, or something else is done to prevent its taking effect. Continuance nisi is a conditional continuance of the case till the next term of the court, unless otherwise disposed of in the mean time.
Nisi prius (Law), unless before; -- a phrase applied to terms of court, held generally by a single judge, with a jury, for the trial of civil causes. The term originated in a legal fiction. An issue of fact being made up, it is, according to the English practice, appointed by the entry on the record, or written proceedings, to be tried by a jury from the county of which the proceedings are dated, at Westminster, unless before the day appointed (nisi prius) the judges shall have come to the county in question (which they always do) and there try the cause. See In banc, under Banc.
Wikipedia
Nisi prius 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.
Usage examples of "nisi prius".
In England, the judges at nisi prius express their opinions freely on the value and weight of the evidence, and the judges in banc, by consent of parties, constantly draw inferences of fact.
Cross, /3/ Lord Ellenborough ruled at nisi prius that a lender could maintain trespass for damage done to a chattel in the hands of a borrower, and that the case is often cited as authority without remark.
But this supposition not only fails to account for Shakespeare's peculiar freedom and exactness in the use of that phraseology, it does not even place him in the way of learning those terms his use of which is most remarkable, which are not such as he would have heard at ordinary proceedings at NISI PRIUS, but such as refer to the tenure or transfer of real property, 'fine and recovery,' 'statutes merchant,' 'purchase,' 'indenture,' 'tenure,' 'double voucher,' 'fee simple,' 'fee farm,' 'remainder,' 'reversion,' 'forfeiture,' etc.
Atqui talia contuleramus arma, quae nisi prius abiecisses, inuicta te firmitate tuerentur.