What is Nisi?

Legal Definition
This word is frequently used in legal proceedings to denote that something has been done, which is to be valid unless something else Shall be done within a certain time to defeat it. For example, an order may be made that if on the day appointed to show cause, none be shown, an injunction will be dissolved of course, on motion, and production of an affidavit of service of the order. This is called an order nisi. Ch. Pr. 547. Under the compulsory arbitration law of Pennsylvania, on the filing of the award, judgment nisi is to be entered: which judgment is to be as valid as if it had been rendered on the verdict of a jury, unless an appeal be entered within the time required by the law.
-- Bouviers Law Dictionary
Legal Definition
Lat. Unless. The word is often affixed, as a kind of elliptical expression, to the words "rule," "order," "decree," "judgment," or "confirmation," to indicate that the adjudication spoken of is one which is to stand as valid and operative unless the party affected by it shall appear and show cause against it or take some other appropriate step to avoid it or procure its revocation. Thus a "decree nisi" is one which will definitely cenclude the defendant's righis unless, within the prescribed time, he shows cause to set it aside or suceessfully appeals. The word, in this sense, is opposed to "absolute." And when a rule nisi is finally confirmed, for the defendant's failure to show cause against it, it is said to be "made absolute."
See also
-- Black's Law Dictionary
Legal Definition
"Unless". A decree that does not enter into force unless some other specified condition is met.
Legal Definition
Unless; if not.
-- Ballentine's Law Dictionary