adverb, noun
all day long; throughout the day
Describes an action or state that continues for the entire day. Often used with verbs like いる, 寝る, 働く, or 遊ぶ.
昨日は一日中雨が降っていた。
It rained all day yesterday.
彼は一日中ゲームをして過ごした。
He spent the whole day playing games.