Real Dictionary

metaphor
variable noun: A metaphor is an imaginative way of describing something by referring to something else which is the same in a particular way. For example, if you want to say that someone is very shy and frightened of things, you might say that they are a mouse.
variable noun: If one thing is a metaphor for another, it is intended or regarded as a symbol of it.

metaphor in American English
a figure of speech containing an implied comparison, in which a word or phrase ordinarily and primarily used of one thing is applied to another (Ex.: the curtain of night, “all the world's a stage”)
noun: a figure of speech in which a term or phrase is applied to something to which it is not literally applicable in order to suggest a resemblance, as in “a mighty fortress is our God”

metaphor in British English
noun: a figure of speech in which a word or phrase is applied to an object or action that it does not literally denote in order to imply a resemblance, for example he is a lion in battle