Midnight; the dead of night—typically the late-night hours around when the date changes. Can also be used adverbially.
真夜中に大きな物音がして目が覚めた。
A loud noise woke me up in the middle of the night.
彼は真夜中まで仕事をしていた。
He was working until midnight.
真夜中の散歩は静かで気分が落ち着く。
A midnight walk is quiet and calming.