彻底 (chè dǐ) means 'thorough' or 'complete'—it describes something done fully, without leaving anything undone or unexamined. The character 彻 (chè) originally means 'to penetrate' or 'to go through', while 底 (dǐ) means 'bottom' or 'base'; together, they evoke the image of going all the way to the very bottom, leaving no part untouched. This vivid imagery underscores its core sense of absolute completeness.

彻底 is commonly used with verbs (e.g., 彻底解决 'thoroughly resolve', 彻底改变 'completely change') and can function as either an adjective (modifying a noun: 彻底的改革 'a thorough reform') or an adverb (modifying a verb: 彻底弄清楚 'figure out completely'). It often appears in formal contexts—policy documents, academic writing, or serious discussions—but also occurs naturally in everyday speech when emphasizing total resolution or finality.