Who is revealed to be the Half-Blood Prince in the Harry Potter books?

Severus Snape is revealed to be the Half-Blood Prince in the Harry Potter books.

The title refers to Snape’s annotated copy of Advanced Potion-Making, which Harry uses during his sixth year. The book contains clever brewing improvements and handwritten spell instructions, including Sectumsempra. Its owner identifies himself as the Half-Blood Prince.

Harry initially assumes the title belongs to someone else and does not know the owner’s identity. The revelation comes after Harry discovers that Snape created the notes and spells. “Half-Blood” reflects Snape’s parentage: his mother, Eileen Prince, was a witch, while his father, Tobias Snape, was a Muggle. “Prince” comes from his mother’s family name.

The title is not another name for Voldemort. Tom Riddle was a half-blood wizard, but he was never the Half-Blood Prince.