Which term is used to describe the greatest achievement of an artist or writer?
And the answer: magnum opus.
In Latin, magnum means "great," and opus means "work," so magnum opus refers to someone's greatest work. Although the term often refers to literature, it's also used to describe other kinds of great works, including paintings, movies, and even construction projects.
An artist, author, or filmmaker can innovate successfully over the course of their entire career, but it takes a special convergence of hard work, skill and circumstance to produce the body of work that will go on to define their legacy. Thus, the magnum opus is often characterized by the maximization of opportunity that takes place at an individual's creative peak. The Sistine Chapel's ceiling is considered Renaissance artist Michelangelo's magnum opus, while Einstein's magnum opus is his Theory of Relativity.
The phrase magnum opus is closely aligned with "masterpiece," a more well-known way to describe the greatest work of an individual. Originally, the term "masterpiece" referred to a piece of work produced by an apprentice aspiring to become a master craftsman in the old European guild system. Fitness to qualify for guild membership was judged partly by the masterpiece, and if the apprentice was successful, the piece was retained by the guild. Great care was therefore taken to produce a fine piece in whatever the craft was, whether confectionery, painting, goldsmithing, knifemaking, leatherworking, or many other trades.
Today, though, a masterpiece or magnum opus can be used to describe the greatest creation by an individual across a broad swath of disciplines and vocations – it's not limited to a physical trade.