From Wikipedia on 11-Nov-2012
A metaphor is a literary figure of speech that describes a subject by asserting that it is, on some point of comparison, the same as another otherwise unrelated object. Metaphor is a type of analogy and is closely related to other rhetorical figures of speech that achieve their effects via association, comparison or resemblance including allegory, hyperbole, and simile.
One of the most prominent examples of a metaphor in English literature is the All the world’s a stage monologue from As You Like It:
All the world’s a stage,
And all the men and women merely players;
They have their exits and their entrances;
— William Shakespeare, As You Like It, 2/7
This quote is a metaphor because the world is not literally a stage. By figuratively asserting that the world is a stage, Shakespeare uses the points of comparison between the world and a stage to convey an understanding about how the world works and the lives of the people within it.
Etymology
The English metaphor derives from the 16th c. Old French métaphore, from the Latin metaphora “carrying over” from the Greek (μεταφορά) metaphorá “transfer”, from (μεταφέρω) metaphero “to carry over”, “to transfer” and from (μετά) meta “between” + (φέρω) phero, “to bear”, “to carry”.
Comparison with other types of analogy
Metaphors are most frequently compared with similes. The Colombia Encyclopedia, 6th edition, explains the difference as:
a simile states that A is like B, a metaphor states that A is B or substitutes B for A.
Where a metaphor asserts the two objects in the comparison are identical on the point of comparison, a simile merely asserts a similarity. For this reason a metaphor is generally considered more forceful than a simile.
The metaphor category also contains these specialised types:
allegory: An extended metaphor wherein a story illustrates an important attribute of the subject.
catachresis: A mixed metaphor used by design and accident (a rhetorical fault).
parable: An extended metaphor narrated as an anecdote illustrating and teaching a moral lesson, such as Aesop’s fables.
Metaphor, like other types of analogy, can usefully be distinguished from metonymy as one of two fundamental modes of thought. Metaphor and analogy both work by bringing together two concepts from different conceptual domains, whereas metonymy works by using one element from a given domain to refer to another closely related element. Thus, a metaphor creates new links between otherwise distinct conceptual domains, whereas a metonymy relies on the existing links within them.
Conceptual Metaphor
Some theorists have suggested that metaphors are not merely stylistic, but that they are cognitively important as well. In Metaphors We Live By George Lakoff and Mark Johnson argue that metaphors are pervasive in everyday life, not just in language, but also in thought and action. A common definition of a metaphor can be described as a comparison that shows how two things that are not alike in most ways are similar in another important way. They explain how a metaphor is simply understanding and experiencing one kind of thing in terms of another. The authors call this concept a ‘conduit metaphor.’ By this they meant that a speaker can put ideas or objects into words or containers, and then send them along a channel, or conduit, to a listener who takes that idea or object out of the container and makes meaning of it. In other words, communication is something that ideas go into. The container is separate from the ideas themselves. Lakoff and Johnson give several examples of daily metaphors we use, such as “argument is war” and “time is money.” Metaphors are widely used in context to describe personal meaning. The authors also suggest that communication can be viewed as a machine: “Communication is not what one does with the machine, but is the machine itself.” (Johnson, Lakoff, 1980).