(verb) to set aside or annul a wrongly made order or judgment: "The court refused to vacate the lower court's judgment."

Synonym: set aside: "The Court granted the plaintiff's motion to set aside the jury's verdict."

Antonym:  affirm: "Since the Court of Appeal affirmed the trial court judgment against the plaintiff, the plaintiff decided to appeal to the Supreme Court."

Alternative definition: to move out of real estate so that you no longer occupy that real estate: "The tenant refused to vacate the premises."

Common phrases
The appeals court would be within its power to vacate its ruling.
The court is going to vacate the rules.
Unless they vacate their homes in 48 hours, they will be evicted.
The owners agreed to vacate the property.
The disqualified board member should vacate his seat.
Source: YourDictionary