Nunc pro tunc literally means "now for then."
Occasionally, a court or party to a divorce forgets to file the papers necessary to obtain the final decree (after the interlocutory judgment has been granted), and the result is that the divorce never becomes final. If the oversight presents a problem (for example, one party has already remarried, or there is a tax advantage to being divorced earlier), the court may agree to issue a nunc pro tunc order, which grants the final divorce retroactive to the earlier date.
This phrase is used to express that a thing is done at one time which ought to have been performed at another. Leave of court must be obtained to do things nunc pro tunc, and this is granted to answer the purposes of justice, but never to do injustice. A judgment nunc pro tunc can be entered only when the delay has arisen from the act of the court.