The Collaborative International Dictionary of English v.0.48:

Nisi \Ni"si\, conj. [L.]
   Unless; if not; -- used mostly in law.
   [1913 Webster]

   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.
         [1913 Webster]

   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.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

NISI PRIUS. 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 assizes 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 assizes there. 1 Reeves, 246; 2 Inst. 422, 3, 4. These local
trials being found convenient, were applied not only to assizes, 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 assizes. It is this provision of the statute of Nisi
Prius, enforced by the subsequent statute of 14 Ed. III. c. 16, which
authorizes, in England, a trial before the justices of assizes, 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.