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]

WordNet (r) 3.0 (2006):

nisi
    adj 1: not final or absolute; "the decree is nisi and not
           absolute"

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

NISI
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