An "epic" is defined by Barton and Hudson's A Contemporary Guide to Literary Terms (Houghton Mifflin, 1997) as follows:
"An epic is defined as a long narrative poem 'celebrating episodes of a people's heroic tradition...' In many cases, it is a mixture of myth and mystery.
"Many critics distinguish between two kinds of epics: the traditional (or folk) epic and the literary (or art) epic. The traditional epic, which may have been originally an oral or anonymous poem, typically focuses on the adventures of a hero who is important to or representative of a nation...
"As is often the case in folk epics, the problems of the hero... are caused or exacerbated by the wills and whims of deities... The poem [The Iliad] begins in media res; that is, in the middle of the action... Other formulae include epic catalogues of warriors, long speeches, and epic similes or epithets.
"The literary or art epic is a conscious imitation of a folk epic... Later literary epics often focused on the Christian tradition and the Christian God..."
Even Milton's epic follows many of the conventions of classic epics. For an example, in Book 2, he makes a classical reference to Atlas.