What is the prayer called which traditionally opens an epic poem?

Most epic poems begin with an invocation, which is a prayer to some higher power. The poet generally asks the gods, often a Muse, to guide his pen during the writing of the poem. In Greek epic poetry, the Muse addressed was usually Calliope, who was the Muse specifically of epic poetry. At other times, the poet would ask another deity for inspiration, but the purpose of the invocation was to introduce the action that will unfold in the poem.