absentmindedness

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

1. His absentmindedness often caused him to misplace his keys.
   他的心不在焉常常导致他把钥匙放错地方。
2. She apologized for her absentmindedness during the meeting, as she had been preoccupied with other thoughts.
   她为在会议期间的心不在焉道歉，因为她一直在思考其他事情。