Half a day — about 12 hours, or roughly one of the two halves of a day (morning or afternoon). Can be used as a noun or adverb.
打ち合わせは半日で終わったので午後は自由だった。
The meeting finished in half a day, so the afternoon was free.
半日観光のツアーに参加して有名な寺を見てきた。
I joined a half-day sightseeing tour and saw a famous temple.
子どもが熱を出したので今日は半日で早退した。
My child developed a fever, so I left early after half a day today.