Metaphor is a figure of speech in which one thing is described in terms of another — not as comparison (that is simile) but as identification. “The road was a ribbon of moonlight” does not say the road resembles a ribbon; it says the road is a ribbon, and the reader holds both terms simultaneously.
Aristotle considered metaphor the mark of genius — “the one thing that cannot be learnt from others” — because it requires perceiving similarity between dissimilar things [@aristotle_poetics]. This overstates the case; metaphor is a learnable craft skill. But the insight is sound: metaphor works by connecting domains that don’t obviously belong together, and the surprise of the connection is part of its meaning.
Metaphors operate at multiple scales. At the smallest, they are local figures: “drowning in paperwork.” At the largest, they organize entire poems or narratives — the journey as a metaphor for life, the garden as a metaphor for cultivation of the self. Extended metaphors (also called conceits) sustain a single comparison across a passage or entire work.
Dead metaphors — “the leg of the table,” “the mouth of the river” — have lost their figurative charge through familiarity. George Orwell warned against using dead or dying metaphors precisely because they no longer create the perceptual connection that makes metaphor work: the writer reaches for them without thinking, and the reader receives them without seeing [@orwell1946].