The term masterpiece is one which has gained many connotations through overuse in contemporary society.

Originally (and still in certain circles) the word was used to describe the work created by an apprentice to demonstrate the skills and knowledge learned. Every last bit of the apprentice's energy and patience would be put into this work, making it the best one (so far) of his (not often her) career. After completing a masterpiece, the apprentice would be able to market his (or her) own skills and begin training apprentices as a recognized master. It was the way of things among those who worked any trade -- from art to metalwork to carpentry. Think of the master's thesis one produces during graduate school: After completing this daunting task, one has survived a "trial by fire" and generally receives certification (in the form of a degree) stating that one is qualified to not only practice in their field but teach in it as well.

Michaelangelo's Pieta is one of the best-known masterpieces. He was so proud of this work that he vowed never to finish another, so as to prevent later pieces from possibly being interpreted as "better"!

Of course, nowadays (and apparently even in Webster's 1913 days, below) the term applies to virtually any critically-acclaimed work of art and is even used to describe works which would theoretically be well before or after the creator's passage from learner to teacher (assuming the transition is made at all). Check any newspaper's movie reviews for a good example.