Why is The Odyssey an epic poem?

There are many definitions of what an epic poem is.
However, all poets and academics agree that the Odyssey is one of the classic examples
of epic poetry.

To be classed as an epic, a poem must be
long. But how long?
often book length, but
there's no set length for which a poem goes from non-epic to
epic.

Does an epic need
form?
No, but they are often in verse form. As long as they are
a narrative. Think of epic poems as a way to pass on historical events, in an
entertaining, oral, fashion.
Both the Illiad and the Odyssey are in dactylic
hexameter, but this is not a requirement of epics. However, it is the usual form of
Greek and Latin epic poetry.

Elements often
included in epics:
superhuman deeds, heroic adventures.
Illustrative language - remember the first audience for epics were listeners, rather
than readers.

How have epics
changed?
Lord Byron and Alexander Pope both wrote epics, but
utilised them for comic effect. Twentieth century poets rekindled interest in the long
form, but the work of (e.g.) Charles Olsen and Frank Standford is usually described as a
're-envisioning' of the epic style.