下界

xiàjiè
  1. lower bound (math.)
  2. world of mortals
  3. (of gods) to descend to the world of mortals

CEDICT

math

Entry Composition

1 下 xià
  1. down
  2. downwards
  3. below
  4. lower
  5. later
  6. next (week etc)
  7. second (of two parts)
  8. to decline
  9. to go down
  10. to arrive at (a decision, conclusion etc)
  11. measure word to show the frequency of an action
2 界 jiè
  1. (bound form) boundary; border
  2. (bound form) realm