The standard definition of an epic, or heroic poem, is that it is a ''noble story told in noble verse'' (Hutson and McCoy, Epics of the Western World, p. 7), a continuous narrative concerning a heroic person from history or tradition.
The epic is an extended narrative dealing with a hero or group of heroes attempting to achieve a specific goal. This goal frequently has to do with actions, events, or ideas that tend to define a culture either through history, values, or destiny, or, at times, all three. Any poem can be heroic, but the epic is separated from other heroic narratives through its magnitude and style. In simplest terms, epics are very long and written in a highly elevated style.
The fate of humankind thus becomes the unifying force of the poem, as Milton presents the ideals of private virtue and public rectitude by exploring both the nobility and weakness of fallen humanity.
Like the classical epic, Paradise Lost alternates its setting between the world of men and the worlds of God and the angels (fallen and unfallen). As well as the typical epic forms described above, a number of epic motifs are incorporated by Milton into the poem. For example, he incorporates mythology, though it is Biblical, not classical, myth which dominates Paradise Lost.
Although he claims that war, the traditional subject matter of the epic, is not to be his theme, he does incorporate the motif of battle into the poem.
An epic is a story that relates not only to a specific group of society, geography, or geneology, but one that can speak to all mankind. Milton could have chosen to write this poem as a commentary on England or as a message to Europe, but it is quite clear that this "epic" is directed towards all mankind. In fact, it even goes outside the walls of our own species and addresses the entire spiritual realm. This makes it truly epic because it will never not apply to any person. The story will forever be applicable to every branch of humankind.