The Bedford Glossary of Critical and Literary Terms - Ross Murfin 2018

Invocation

Invocation: A type of apostrophe in which a direct and explicit request for help in writing (usually verse) is made to a divine or supernatural entity. Classical convention called for such an address to the muses; in the epic, the address was typically directed to Calliope, the muse of epic poetry, or to Clio, the muse of history. Invocations remained relatively common through the Neoclassical Period but are rare today.

EXAMPLE: At the beginning of his cosmic seventeenth-century epic Paradise Lost (1667), John Milton invoked Urania, the muse of astronomy in antiquity, remade by Christian writers of the Renaissance into a heavenly, Christian muse.