The word "一旦" is formed through a conceptual fusion where the character "一" (one) signifies a singular, specific point, and "旦" (dawn) represents the break of day, a distinct, pivotal moment of transition from night to day. By combining them, the term metaphorically abstracts the idea of a particular, defined moment, like the instant the sun rises, to signify a critical juncture or a hypothetical point in time, creating a sense of a decisive, often unforeseen, turning point.