一旦

yīdàn
  1. in case (sth happens)
  2. if
  3. once (sth happens, then...)
  4. when
  5. in a short time
  6. in one day

CEDICT

Entry Composition

1 一 yī
  1. one
  2. single
  3. a (article)
  4. as soon as
  5. entire; whole; all; throughout
  6. "one" radical in Chinese characters (Kangxi radical 1)
  7. also pr. yāo for greater clarity when spelling out numbers digit by digit
2 旦 dàn
  1. (literary) dawn; daybreak
  2. dan, female role in Chinese opera (traditionally played by specialized male actors)