Metaphor (from the Greek: μεταφορά - metaphora, meaning "transfer") is language that directly compares seemingly unrelated subjects. In the simplest case, this takes the form: "The subject is a subject." More generally, a metaphor is a rhetorical trope that describes a first subject as being or equal to a second object in some way. Thus, the first subject can be economically described because implicit and explicit attributes from the second subject are used to enhance the description of the first. This device is known for usage in literature, especially in poetry, where with few words, emotions and associations from one context are associated with objects and entities in a different context.

A metaphor is generally considered to be more forceful and active than an analogy (metaphor asserts two topics are the same whereas analogy may acknowledge differences. Other rhetorical devices involving comparison, such as metonymy, synecdoche, simile, allegory and parable, share much in common with metaphor but are usually distinguished by the manner in which the comparison between subjects is delivered.

Structure

The metaphor, according to I. A. Richards in The Philosophy of Rhetoric (1936), consists of two parts: the tenor and vehicle. The tenor is the subject to which attributes are ascribed. The vehicle is the subject from which the attributes are borrowed. Other writers employ the general terms ground and figure to denote what Richards identifies as the tenor and vehicle. Consider: All the world's a stage:-

''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 well-known quotation is a good example of a metaphor. In this example, "the world" is compared to a stage, the aim being to describe the world by taking well-known attributes from the stage. In this case, the world is the tenor and the stage is the vehicle. "Men and women" are a secondary tenor and "players" is the vehicle for this secondary tenor.

The corresponding terms to 'tenor' & 'vehicle' in alternate viewpoint terminology are target and source. In this nomenclature, metaphors are named using the typographical convention "TARGET IS SOURCE", with the domains and the word "is" in small capitals (or capitalized when small-caps are not available); in this notation, the metaphor discussed above would state that "LIFE IS THEATRE". In a conceptual metaphor the elements of an extended metaphor constitute the metaphor's mapping--in the Shakespeare passage above, for example, exits would map to death and entrances to birth.