The term "master work" refers to a piece of art or craftsmanship that is considered to be the greatest achievement of an artist or artisan. There are several synonyms for this phrase that can be used interchangeably, depending on the context in which it is being used. For example, "tour de force" or "magnum opus" are commonly used in the context of art, literature, music, or film. "Definitive work" or "seminal work" are more suited to describe works that have had a significant impact on a particular field or genre. Additionally, "masterpiece" or "chef-d'oeuvre" are other terms that can be used to describe a work of art that is considered to be the artist's greatest accomplishment.