A metaphor is a figure of speech that, for rhetorical effect, directly refers to one thing by mentioning another. It may provide (or obscure) clarity or identify hidden similarities between two different ideas. Metaphors are often compared with other types of figurative language, such as antithesis, hyperbole, metonymy, and simile. One of the most commonly cited examples of a metaphor in English literature comes from the "All the world's a stage" monologue from As You Like It:

This quotation expresses a metaphor because the world is not literally a stage, and most humans are not literally actors and actresses playing roles. By asserting that the world is a stage, Shakespeare uses points of comparison between the world and a stage to convey an understanding about the mechanics of the world and the behavior of the people within it.

In the ancient Hebrew psalms (around 1000 B.C.), one finds already vivid and poetic examples of metaphor such as, "The Lord is my rock, my fortress and my deliverer; my God is my rock, in whom I take refuge, my shield and the horn of my salvation, my stronghold" and "The Lord is my shepherd, I shall not want". Some recent linguistic theories view all language in essence as metaphorical.

The word metaphor itself is a metaphor, coming from a Greek term meaning "transference (of ownership)". The user of a metaphor alters the reference of the word, "carrying" it from one semantic "realm" to another. The new meaning of the word might be derived from an analogy between the two semantic realms, but also from other reasons such as the distortion of the semantic realm - for example in sarcasm.