Lat Unless. The word is often affixed, as a kind of elllptlcal expression, to the words “rule,” “order,” “decree,” “Judg-ment,” or “confirmation,” to Indlcate that the adjudication spoken of is one’ which is to stand as valid and operative unless the par-ty affected by it shall appear and show cause against it, or take some other appro-priate step to avoid It or procure Its revoca-tion. Thus a “decree nisi” is one which will definitely conclude the defendant’s rlghts unless, within the prescribed time, he shows cause to set it aside or successfully appeals. The word, in this sense, is opposed to “ab-solute." And when a rule nisi is finally con-firmed, for the defendant’s failure to show cause against it, it is said to be “made abso-lute.”
Source: Black's Law Dictionary 2nd Ed (1910)
