A metaphor is a figure of speech in which a term or phrase is applied to something to which it is not literally applicable. This is done in order to suggest a resemblance: "She is a perfect lamb." Metaphor and simile are allied in meaning; a simile expresses resemblance directly but does so by using as, as if, like: "She is as sweet as a flower." Unfortunately, most metaphors and some similes are either strained or trite. Many figures of speech are often mixed; standard advice is to sustain one figure and not suddenly shift to another: "We had the crankcase drained and thus nipped our trouble in the bud."
Dictionary of problem words and expressions. Harry Shaw. 1975.