The Caspian Sea is the world's largest lake by surface area, even though its traditional name calls it a sea. It covers roughly 371,000 square kilometres and lies in an endorheic basin between Europe and Asia.
Its classification is a frequent source of debate. The Caspian is saline, has no natural outlet to the world ocean, and is conventionally treated as the largest lake. Geologically, however, it occupies part of an ancient oceanic basin and is sometimes described as a small ocean or inland sea. For ordinary geography quizzes, “Caspian Sea” is the canonical answer.
The main mix-up is with North America's Great Lakes. Lake Superior is the largest lake entirely within a single country and the largest of the Great Lakes by area, while Lake Michigan–Huron can be counted as one hydrologically connected lake. Neither convention displaces the Caspian in the usual worldwide ranking.