1) Historically, a piece of work that an artist or craftsman, having finished his apprenticeship, submitted to his guild to gain the status of master.

2) In current usage, a work of art of outstanding quality, or the greatest work produced in an artist’s career.

“New York City—Amateurs Copying the Masterpieces of Celebrated Artists: Scene in the Metropolitan Museum of Art.” Frank Leslie’s Illustrated Newspaper. May 26, 1888.