Metaphor is the umbrella term for a series of figurative “tropes” which, like irony, use one aspect of language to stand in for and refer to another, bestowing a subtly different meaning because of its presence: simile, metaphor, metonym, personification, and synecdoche are all examples of tropes. Simile is an informational trope (using “like” or “as”) that lets us know about an unfamiliar word by describing it in more familiar terms: In Wilfred Owen’s “Anthem for Doomed Youth,” how do the soldiers die? As cattle being ignominiously slaughtered. Personification performs a similar role, though it functions by applying human characteristics to any concepts or things that are non-human, such as April being the “cruelest month” at the beginning of T. S. Eliot’s The Waste Land (actually, this last example of personification is even more specific; personification used to describe inanimate nature is further defined as the “pathetic fallacy,” after the term invented by the nineteenth-century critic John Ruskin). Metonym is another trope in which something is referred to by the use of another concept which is commonly and physically associated with it. For example, if I were a royal tax collector, I could say I collect taxes on behalf of the crown (if we lived in a kingdom), because kings wear crowns. But you would not be confused about suddenly owing taxes to headgear. Finally, synecdoche is a subspecies of metonymy in which you refer to the whole by use of a part, such as in the naval expression, “all hands on deck!” (the expectation is that the sailors will appear on deck with all body parts intact).
Proper metaphors do not have the “like” or “as” apparatus that the simile sports; they simply make an equation of two unlikely things: “He is a doormat,” where the “he” being troped is someone who lets people walk all over him; or, “She is a gazelle,” where “she” is exceptionally fast, like a gazelle (or exceptionally slow, if it’s a metaphor being used as irony). The word (the image, activity, or concept) that appears within the text is called the vehicle (doormat; gazelle); the other idea, activity, or concept it has substituted itself for is known as the tenor (pushover; speedy). Again, the vehicle is the thing you point to within the text, and the tenor is the idea that you infer. A proper metaphorical pair of tenor and vehicle has a clear principle of substitution; no one mistakes the fact that the vehicle is meant to convey an important characteristic about the tenor, rather than describing some bizarre metamorphosis. But we shouldn’t ignore the consequences of the vehicle’s presence. In terms of content, we assimilate the information about the tenor and move on; but the presence of the vehicle often suggests resonances that cannot be dismissed. Sometimes we may begin thinking the tenor-vehicle coupling is informational but arbitrary, but then later learn there was more of a relationship than we had first suspected between the two (then the pair resembles metonymy instead). When the vehicle might stand in for two tenors, we have an instance of ambiguity. When the vehicle stands in for something, but it’s impossible to know precisely what, that is called indeterminacy (it’s extremely rare). When the substitution is so common that the vehicle is conventional and familiar, that metaphor is also known as a symbol. Here, the associative power of language trumps the text’s attempt to create an instantaneous, new, idiosyncratic coupling of a tenor and vehicle; a symbol is a metaphor whose resonances rely as much or more on the reader than the text to provide its meaning.
In each case of metaphor, be sure to specify the relationship between the tenor and vehicle, and the consequences of the use of the particular vehicle to stand in for the tenor. Kings often wear gold crowns, but there may be nothing particularly golden about the king himself. Calling “all hands on deck” instead of “all toes on deck” probably has something to do with the type of skilled work that is required. And the use of either “doormat” or “gazelle” carries with it a negative or positive connotation that we should note.