How is "metaphor" defined?

Metaphor is where one idea is used to talk about something else. The metaphor is something about which you or your listener already knows more, while you know less about the “something else” to which the metaphor is applied.  The effect of the metaphor is to transfer the understanding from the better-known “source” to the lesser-known “target”.

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Next:     You can approach metaphor in many ways.
What line is taken here?