something to think about; one's thoughts; concern; worry
thinking about something; being absorbed in one's thoughts
彼は考え事にふけっていた。
He was deep in thought.
顔色が悪いけど、何か考え事があるの？
You look pale; do you have something on your mind?
電車の中で考え事をしていて、乗り過ごしてしまった。
I was lost in thought on the train and missed my stop.