I checked the entry "Epic fantasy" in The Encyclopedia of Fantasy, and the definition and comments are very interesting:
"An epic is a long narrative poem which tells large tales, often incorporating a mixture of legend, myth and folk history, and featuring heroes whose acts have significance transcending their own individual happiness or woe. The classic epic tells the story of the founding or triumph of a a folk or nation."
Examples: Homer's Iliad and Odyssey, Beowulf.
More pertinent for this discussion is the question of contemporary prose epic fiction (fantasy). The examples given are: Morris, E.R. Eddison, Tolkien, and Donaldson. (At least I got three on my list!
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The article goes on to state that "any fantasy tale written to a large scale which deals with the founding or definitive and lasting defence of a Land may fairly be called an Epic Fantasy."
"Unfortunately, the term has been increasingly used by publishers to describe Heroic Fantasies that extend over several volumes, and has thus lost its usefulness." - So, there we go with Robert Jordan again ...! 