metaphor [Gr.,=transfer], in rhetoric, a figure of speech in which one class of things is referred to as if it belonged to another class. Whereas a simile states that A is like B, a metaphor states that A is B or substitutes B for A. Some metaphors are explicit, like Shakespeare's line from As You Like It: "All the world's a stage." A metaphor can also be implicit, as in Shakespeare's Sonnet LXXIII, where old age is indicated by a description of autumn:

That time of year thou mayst in me behold
  Where yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
  Bare ruined choirs, where once the sweet birds sang.

A dead metaphor, such as "the arm" of a chair, is one that has become so common that it is no longer considered a metaphor.

Figure of speech in which a word or phrase denoting one kind of object or action is used in place of another to suggest a likeness or analogy between them (as in “the ship plows the seas” or “a volley of oaths”). A metaphor is an implied comparison (as in “a marble brow”), in contrast to the explicit comparison of the simile (“a brow white as marble”). Metaphor is common at all levels of language and is fundamental in poetry, in which its varied functions range from merely noting a likeness to serving as a central concept and controlling image.

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See Metaphor