hundred days; lengthy time
Literally 100 days; often used figuratively to mean a long period of time, especially in set phrases like 百日の説法屁一つ (a single fart ruins a hundred days of preaching).
I continued practicing every day for a hundred days.
A hundred days of hard work came to nothing.
A plant name (crape myrtle) that uses the same kanji but has a completely different reading and meaning.
Compound of 百 (ひゃく, 'hundred') and 日 (にち, 'day'). The figurative sense of 'lengthy time' arises naturally from the literal meaning.