There are broad categories of figurative language which are classified as metaphorical (see Literal and figurative language). The more common meaning of metaphor is a figure of speech that is used to paint one concept with the attributes normally associated with another. "X is a metaphor for Y" means that Y is painted with the attributes of X.
All metaphors can be analyzed and reduced to the equation "X equals Y." Examples in everyday languge abound.
The expression, "You are the sunshine of my life" equates someone's beloved with sunshine; something that is impossible in literal terms unless that person becomes a ball of nuclear fusion. The expression "candle in the wind" likens life's fragility to an extinguished candle.
"Life in the fast lane" (left lane of freeway = a fast and/or hectic pace), or "bowels of the ship" (intestines = the inner holds of a ship) or "drowning in money" (drowning = having too much), or "beating your head against the wall" (beating your head = taking ineffectual actions; the wall = the problem), or "he's still wet behind the ears" (he has not completely dried yet, he's still fresh/new), or "Luca Brasi sleeps with the fishes" (sleeping where fish live = being dead).
In political discussions, a ship is often taken as a metaphor for an entire nation; the so-called "ship of state". This metaphor likens a nation to a ship. It implies that, just as a ship needs a captain to make decisions, give orders, and coordinate and control the ship's voyage, so a nation must have a government to coordinate and control the nation's business. Therefore, referring to a nation as a "ship of state," emphasizes the need that nations have for some kind of government.
Metaphor is usually distinguished from simile. Both compare two seemingly unrelated objects, but, in the latter, the comparison is made more explicit, usually through the use of the words "like" or "as". "Life is but a dream" is a metaphor, for example, while "getting money from him is like pulling teeth" is a simile.
Metaphor is one of the most common figures of speech and many words have their origin in metaphor. When a metaphor is so common that people usually take it for granted, it is called a dead metaphor. Understanding, for example, is a dead metaphor, having its origins in the idea that "standing under" something was akin to having a good grasp of it (another, slightly less dead metaphor) or knowing it thoroughly.
Metaphors are seen as very powerful tools because they allow for the expression of abstract principles by reference to concretes. They can also be dangerous to understanding, in that people may fail to recognize the figurative nature of a metaphor, and come to take it literally.
On the other hand, since so many, many words are dead metaphors, attempting to avoid them entirely would end in silence. For instance, consideration is a metaphor meaning "take the stars into account", mantel means "cloak or hood to catch smoke", gorge means throat, and so forth for thousands more.
The mixed metaphor entails using two living metaphors in obvious conflict, such as: "That wet blanket is a loose cannon"; "Strike while the iron is in the fire"; or (said by an administrator whose government-department's budget was slashed) "Now we can just kiss that program right down the drain". On the other hand, to refer to a photograph enlarged too much as "a grainy shot" is not a mixed metaphor, even though both the grain and the shot were originally metaphorical.
Many consider metaphor to be at the heart of poetry (or even to define in part what it means to be human): the figure of speech that links dissimilar objects for their resemblance. For example, Emily Dickinson uses "the white assassin" as a metaphor for frost. Ground may have a blanket of snow where blanket is a metaphor for cover.
Originally, metaphor was a Greek word meaning "transfer". The Greek etymology is from meta, implying "a change" and pherein meaning "to bear, or carry". Thus, the word metaphor itself has a metaphorical meaning in English, "a transfer of meaning from one thing to another".