n.preoccupation so great that the ordinary demands on attention are ignored

1. His absentmindedness often caused him to misplace his keys.
   他的心不在焉常常導致他把鑰匙放錯地方。
2. She apologized for her absentmindedness during the meeting, as she had been preoccupied with other thoughts.
   她為在會議期間的心不在焉道歉，因為她一直在思考其他事情。