The word 一流 is formed through a conceptual metaphor where the character 一 (yī), meaning "one" or "first", is combined with 流 (liú), meaning "flow", "stream", or "class". It visualizes a hierarchy as a flowing river or a series of streams, where the "first flow" or topmost stream represents the highest echelon or class. Thus, the term inherently carries the idea of being "first-class" or "top-tier" by picturing it as the foremost level in a natural, ordered progression.