More generally, a metaphor is a rhetorical trope that describes a first subject as being or equal to a second subject 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.
Within the non rhetorical theory a metaphor is generally considered to be a concluded equation of terms that is more forceful and active than an analogy, although the two types of tropes are highly similar and often confused. One distinguishing characteristic is that the assertiveness of a metaphor calls into question the underlying category structure, whereas in a rhetorical analogy the comparative differences between the categories remain salient and acknowledged. Similarly, metaphors can be distinguished from other closely related rhetorical concepts such as metonymy, synecdoche, simile, allegory and parable.
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 terms ground and figure to denote what Richards identifies as the tenor and vehicle. Consider:
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.