‘一刻’ literally combines ‘一’ (yī, meaning 'one') and ‘刻’ (kè, an ancient Chinese unit of time equal to 15 minutes). Historically, a day was divided into 96 ke, each lasting 15 minutes—hence ‘一刻’ means exactly a quarter-hour. It is still used today in both precise time-telling and figurative expressions.

In modern Mandarin, ‘一刻’ functions as a noun meaning either the literal 15-minute interval or, more commonly, a brief, indeterminate moment—similar to 'an instant' or 'a split second'. Unlike ‘一分钟’ (one minute), ‘一刻’ carries a slightly literary or emphatic tone, especially in set phrases like ‘一刻也不能耽误’ (not a moment can be delayed). It rarely appears in casual speech on its own for time-telling; instead, it’s embedded in fixed expressions or formal contexts.