Severus Snape is revealed in J.K. Rowling’s books to be the Half-Blood Prince.
The revelation is central to Harry Potter and the Half-Blood Prince, the sixth novel. Harry uses an old Potions textbook containing unusually effective instructions and handwritten notes signed “The Half-Blood Prince.” The identity is disclosed near the end of the story as Severus Snape, Hogwarts’ Potions professor at the beginning of the series.
“Half-Blood” refers to Snape’s parentage: his mother was a witch, Eileen Prince, while his father, Tobias Snape, was a Muggle. He combined part of his mother’s surname with his mixed magical heritage to create the title. The name also reflects the book’s wider focus on Snape’s past and his complicated relationship with Harry.