Main Difference
The main difference between Idiom and Metaphor is that the Idiom is a combination of words that has a figurative meaning and Metaphor is a figure of speech.
Idiom
An idiom (Latin: idiomī, “special property”, from Ancient Greek: ἰδίωμα, translit. idíōma, “special feature, special phrasing, a peculiarity”, f. Ancient Greek: ἴδιος, translit. ídios, “one’s own”) is a phrase or an expression that has a figurative, or sometimes literal, meaning. Categorized as formulaic language, an idiom’s figurative meaning is different from the literal meaning. There are thousands of idioms, occurring frequently in all languages. It is estimated that there are at least twenty-five thousand idiomatic expressions in the English language.
Metaphor
A metaphor is a figure of speech that directly refers to one thing by mentioning another for rhetorical effect. It may provide clarity or identify hidden similarities between two ideas. Antithesis, hyperbole, metonymy and simile are all types of metaphor. One of the most commonly cited examples of a metaphor in English literature is the “All the world’s a stage” monologue from As You Like It:
This quotation expresses a metaphor because the world is not literally a stage. By asserting that the world is a stage, Shakespeare uses points of comparison between the world and a stage to convey an understanding about the mechanics of the world and the behavior of the people within it.
The Philosophy of Rhetoric (1937) by rhetorician I. A. Richards describes a metaphor as having two parts: the tenor and the vehicle. The tenor is the subject to which attributes are ascribed. The vehicle is the object whose attributes are borrowed. In the previous example, “the world” is compared to a stage, describing it with the attributes of “the stage”; “the world” is the tenor, and “a stage” is the vehicle; “men and women” is the secondary tenor, and “players” is the secondary vehicle.
Other writers employ the general terms ground and figure to denote the tenor and the vehicle. Cognitive linguistics uses the terms target and source, respectively.
Idiom (noun)
A manner of speaking, a mode of expression peculiar to a language, person, or group of people.
Idiom (noun)
A language or language variety; specifically, a restricted dialect used in a given historical period, context etc.
Idiom (noun)
An established expression whose meaning is not deducible from the literal meanings of its component words, often peculiar to a given language.
Idiom (noun)
An artistic style (for example, in art, architecture, or music); an instance of such a style.
Idiom (noun)
A programming construct or phraseology that is characteristic of the language.
Metaphor (noun)
The use of a word or phrase to refer to something that it is not, invoking a direct similarity between the word or phrase used and the thing described (but in the case of English without the words like or as, which would imply a simile); the word or phrase used in this way; an implied comparison.
Metaphor (noun)
The use of an everyday object or concept to represent an underlying facet of the computer and thus aid users in performing tasks.
“desktop metaphor; wastebasket metaphor”
Metaphor (verb)
To use a metaphor.
Metaphor (verb)
To describe by means of a metaphor.
Idiom (noun)
a group of words established by usage as having a meaning not deducible from those of the individual words (e.g. over the moon, see the light).
Idiom (noun)
a form of expression natural to a language, person, or group of people
“he had a feeling for phrase and idiom”
Idiom (noun)
the dialect of a people or part of a country.
Idiom (noun)
a characteristic mode of expression in music or art
“they were both working in a neo-impressionist idiom”
Metaphor (noun)
a figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable
“her poetry depends on suggestion and metaphor”
“when we speak of gene maps and gene mapping, we use a cartographic metaphor”
Metaphor (noun)
a thing regarded as representative or symbolic of something else
“the amounts of money being lost by the company were enough to make it a metaphor for an industry that was teetering”