What is Nisi Prius Writ?

Legal Definition
The old name of the writ of venire, which originally, in pursuance of the statute of Westminster 2, contained the nisi prius clause. Reg. Jud. 28, 75; Cowell.
-- Black's Law Dictionary
Legal Definition
An old writ directing a sheriff to bring the jurors to Westminster unless before that time the justices of assize came into the county to try cases.
-- Ballentine's Law Dictionary