What is the difference between an idiomatic expression and a figure of speech?

Answer 1

Idiom is one form of "figures of speech." An idiomatic expression is an phrase whose meaning can't be understood based on the meaning of its individual words. e.g. "Hold your tongue," or "under the weather" Figure of speech may be idiomatic expressions, or simply use of words diverging from their usual meaning, but also may refer to several different rhetorical devices including metaphor, or repetition of words, rhyming or alliteration, or irony.

Answer 2

Figures of speech are words or group of words which cannot be literally interpreted to give the intended meaning. They make use of figurative language, such as similes and metaphors, and thus their intended/actual meaning transcends (goes above or beyond) the literal meaning.

Idiomatic expressions (idioms) are common phrases or sayings whose meaning cannot be understood by the individual words or elements.

Idioms are a subset of figures of speech.

Essentially, a figure of speech is any unit of speech that cannot be properly understood with a literal interpretation, since figurative language is used. An idiom fits that description entirely, but the definition of an idiom must also include the detail that they are commonly used and thus understood primarily by being previously heard in context by the listener or explained to the listener. Thus every idiom is a figure of speech, but only some figures of speech are idioms.

A figure of speech is usually an example of simile, metaphor, or hyperbole. They are to be interpreted figuratively, rather than literally.

"I'm starving" is an example of a figure of speech. People very often say it, not to mean that they are literally starving, i.e. dying of malnutrition, but to express that they are hungry, and they are emphasising this with a hyperbole that is also a figure of speech.

An example of an idiom is the phrase 'kicked the bucket'. One understands that to mean 'died', despite there being no way to get that meaning from it as a result of the literal meanings of the words (except potentially in an extremely strange context). Thus the phrase is a figure of speech. However, it is also an idiom because it is not only the case that the real meaning of it is impossible to reach from a literal interpretation; it is also the case that the real meaning cannot be seen as a result of similes or metaphors or such figurative devices, and the only way that most people understand the phase is by having heard it before in context or by having had it explained to them before.

If someone had learned English as a second language, they would likely struggle for quite some time with idioms. This is because they might not understand the true meaning of idioms, since that understanding only comes from hearing them used in context often enough. On the other hand, someone who has not only "learned" English in an academic way but who has also spoken it in colloquial and conversational contexts for some time would usually understand idioms quite easily.