A figure of speech in which two dissimilar things are explicitly compared, often introduced with like or as, as in she runs like the wind.
Simile and Metaphor differ only in degree of stylistic refinement. The Simile, in which a comparison is made directly between two objects, belongs to an earlier stage of literary expression; it is the deliberate elaboration of a correspondence, often pursued for its own sake. But a Metaphor is the swift illumination of an equivalence. Two images, or an idea and an image, stand equal and opposite; clash together and respond significantly, surprising the reader with a sudden light.
—Sir Herbert Read English Prose Style (1928)