Read Edit View history 一旦 yīdàn HSK5 once Origin Definitions Defs Strokes Stroke Words Sentences Sents Senses Add sense 一旦 yīdàn once adverb, conjunction · once (something happens); in case; if 一旦开始，就不能停下来。 Edit Delete 一旦 yīdàn short time idiom · in a single day; in a very short time 他多年的心血毁于一旦。 Edit Delete References Add reference No references for this word.