a matter to think about; one's thoughts or worries; something on one's mind.
彼は最近ずっと考え事をしているように見える。
He looks like he's been lost in thought a lot recently.
そんなに考え事ばかりしていると、集中できないよ。
If you keep worrying about things like that, you won't be able to concentrate.
試験のことが考え事の原因だった。
The exams were the cause of my worries.
夕食中も彼女は考え事をしていて、あまり話さなかった。
She was lost in thought even during dinner and didn't talk much.
the state of being absorbed in thought; to be preoccupied or lost in thought.
授業中に考え事をしていて、先生の質問に答えられなかった。
I was daydreaming during class and couldn't answer the teacher's question.
窓の外を見ながら彼は深い考え事にふけっていた。
He stared out the window, lost in deep thought.
考え事をしていたら、電車を乗り過ごしてしまった。
I missed my stop because I was lost in thought.