NISI PRIUS, in Law, a judicial writ which lies in cases where, the jury being impanelled and returned before the justices of the bank, one of the parties requests to have such a writ for the ease of the country, that the trial may take place before the justices in the same county on their coming thither. The purport of a writ of nisi prius is, that the sheriff is thereby commanded to bring to Westminster the men impanelled, at a certain day, before the justices, nisi prius iudicarii domini regis ad assisas capiendas venerint.