Rule absolute

In the context of American law

Is a mandate or court order.

In the context of the old English Reports

Historically English Law struggled to find proper processes for dealing with appeals against jury verdicts.

In the 18th century a practice grew up of applying to the trial judge for a new trial (not the same thing as an appeal) on the basis that the judge had misdirected the jury, or, later, that the verdict was 'against evidence' i.e. that the jury had come to a perverse verdict.

A judge who thought there might be something in this would make a provisional order for a new trial, known as a 'rule nisi' ('nisi' is Latin for unless) and in this context the order meant that there would be a new trial unless a sort of appeal court, usually consisting of all the judges of that division of the court - including the judge who had made the original ruling, ordered otherwise after further argument.

The sittings of the full court of the division, referred to as sitting 'en banc' would be held at the end of each legal Term, when all the judges would spend a week or two in effect reviewing the difficult cases from the previous term.

Counsel for the party complaining about the verdict were said to appear 'in support of the rule'. and the other side would also be represented in further legal argument as to whether the complaints were justified and a new trial was warranted.

At the end of the argument the court would either decide that the original trial was flawed, in which case the rule nisis would become a rule absolute, and a new trial would be ordered, or would decide that the complaints about the original trial were not justified, in which case the rule would be discharged and the original verdict would stand.

This practice continued in common law courts despite the Common Law Procedure Act 1852, which reformed many aspects of common law procedure, but it disappeared IIRC after the implementation of the Judicature Acts 1873-75 which formally merged common law and equity courts and put in place a more rational appellate structure, which survives very largely intact to the present day.