The word 'nisi' can be used as an adjective and when you are using the word in legal terms and it conveys the idea of something that is yet to be final or being absolute. The word is especially used for a kind of judgement that is slated to be final on a particular date. The distinct feature of this kind of judgement is that in that particular period certain specified contingencies can be brought against it and they may invalidate it. You might have heard the word in association with the word 'decree' and the term that can be formed is 'decree nisi'.

The origin of the word can be traced in the Latin language where it is used to convey the idea of if not or unless.