A nisi divorce judgment is a term used to reference a judgment (in the context of divorce a “divorce judgment”) that is not yet final. It is what is called an “intermediate judgment” because it is in between. You are divorced, but not really. It is an “interlocutory judgment.”
As a general matter, a judgement nisi is a conditional judgment:
A decree nisi or rule nisi (from Latin nisi, meaning “unless”) is a court order that does not have any force unless a particular condition is met. Once the condition is met, the ruling becomes a decree absolute (rule absolute), and is binding. – Wikipédia.
Massachusetts still uses the judgment nisi but most states do not. In Massachusetts, the nisi period is about 90 days. After that the divorce will become absolute.
Judgments nisi is a legal rule that is still employed in the UK where divorce nisi and divorce absolute are terms that are regularly used by divorce practitioners. It appears that it takes approximately 6-8 weeks to go from a divorce nisi to a divorce absolute in UK jurisdiction.
Why is the nisi even still being used? Why was it deemed necessary to have a time-out waiting period? Ostensibly to give people an opportunity to change their minds. Can you remarry with a divorce nisi? No. You need to wait for the divorce absolute.