Apostrophe (rhetoric)
Directly addressing a person, present or absent, or an entity: "O Death, where is thy sting?"
Apostrophe interrupts the discourse to directly address someone or something: a person, the reader, the dead, an abstraction ("O Death, where is thy sting?"). The text stops narrating for a moment in order to address — and that change of axis creates immediate intensity.
In lyric poetry it's constitutive of the genre: the poem addresses the beloved, nature, death. In fiction, apostrophizing the reader ("you, reading this") breaks the fourth wall and seals a pact of complicity — a powerful, costly tool, because once broken, the wall doesn't rebuild.
Example
"O Death, where is thy sting? O grave, where is thy victory?"