Bleeding from the nose; a nosebleed. Can refer to minor or heavy bleeding from the nostrils.
授業中に鼻血が出て、ティッシュで止めた。
I got a nosebleed during class and stopped it with a tissue.
冬は乾燥して鼻血が出やすくなる。
In winter the air is dry and nosebleeds become more likely.
興奮して鼻血が出る、という表現は漫画の定番だ。
The trope of getting a nosebleed from excitement is a staple in manga.