Saying one thing by means of another, without a word of comparison: instead of saying that something is like another thing, one says that it is. The image transfers to the subject the qualities of the thing it has been identified with.
In the Odyssey
Telemachus, what word has slipped past the fence of your teeth! A god, if he wishes, easily saves a man, even from far away.
"The fence of your teeth" makes the mouth an enclosure and the words something that should have stayed shut inside. The image gives Telemachus's phrase the weight of a mistake that got out.
Book III: Telemachus in Pylos, Athena, in the figure of Mentor
Come here, much-praised Odysseus, great glory of the Achaeans, hold back your ship to hear the voice of the two of us. For no one has ever passed by here in a black ship without first hearing the honeyed voice that comes from our mouths; but he goes on delighted and knowing more.
Book XII: The Sirens, Scylla, and the cattle of the Sun, the Sirens
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