Simile, n.; pl. Similes.
Etymology: [L., from similis. See Similar.]
(Rhet.)
A word or phrase by which anything is likened, in one or more of its aspects, to something else; a similitude; a poetical or imaginative comparison. A good swift simile, but something currish. Shak.
Shades of meaning
Fernald has no entry of its own for simile; he lists it under allegory, analogy.
Where he writes about it:
In modern usage we may say that an allegory is an extended simile, while a metaphor is an abbreviated simile contained often in a phrase, perhaps in a word. — from allegory
The simile carries its comparison on the surface, in the words as, like, or similar expressions; the metaphor is given directly without any note of comparison. — from allegory