Nosebleed
From Animepedia
A nosebleed is a commonly used expression to show arousal in some genres of anime and manga. It is a cultural reference to a Japanese old wife's tale that says that people with nosebleeds are harboring extremely lewd thoughts. In anime, it is a commonly used cliché/joke to show a character’s immaturity when faced with nudity. In most cases, these nosebleeds are a male-only phenomenon, but exceptions exist.
The nosebleed is a common feature in the comedy, and some less serious forms of romance. It is rarely used in anime that is aimed at older audiences, or serious in tone.