Apostrophe (rhetoric)
An apostrophe is a rhetorical term meaning speech directed in an abstract direction (as "O Death, where is thy sting?"), to a person not present, or to a thing (as "Thou still unravish'd bride of quietness" in "Ode on a Grecian Urn").It is usually introduced by the word "O" (not the exclamation "oh").
See also Apostrophe (mark).