What is Court Of Nisi Prius?

Legal Definition
In American law. Though this term is frequently used as a general designation of anjf court exercising general, original jurisdiction in civil cases, (being used interchangeably with "trial-court,") it belonged as a legal title only to a court which formerly existed in the city and county of Philadelphia, and which was presided over by one of the judges of the supreme court of Pennsylvania. This court was abolished by the constitution of 1874. See Courts or Assize and Nisi Prius.
-- Black's Law Dictionary
Legal Definition
An English court held by two or more commissioners sent out from London on a circuit to hold jury trials.
-- Ballentine's Law Dictionary