Explanation:
Severus Snape is the Half-Blood Prince because he is a half-blood wizard.
Half-blood wizards are wizards whose parents were both Muggles and wizards. They usually have a more limited magical ability than full-blood wizards. He was born to Tobias Snape, a Muggle and Eileen Prince, a witch. In his childhood, he became good friends with Lily Evans, who later became Harry Potter’s mother. However, when he saw how much Lily loved James Potter, another boy in their school, he started to hate her and turned his back on her.