The difference between an idiom and a phrase is that a phrase is a literal element of grammar while an idiom is a figurative element of culture. Both are elements of linguistics and superficially appear similar. This is because both are repeated fragments and strings of words found in whole or partial sentences.
An idiom and a phrase are both basic units of a sentence. The phrase is part of technical grammar and is one of the most basic parts of syntax. A phrase is considered to be one distinct syntax unit. Phrases are present in everyday speech in an ordinary and functional manner, while idioms are ornamentation to that language.
Phrases are functional and direct. They are composed of multiple words, but the most important word is called the head. The head is the stressed word in the sentence; however, a phrase does not have to comprise an entire sentence. Phrases can be a small fragment of a sentence.
A big difference between an idiom and a phrase is that a phrase is a formal part of technical grammar, while an idiom is more inventive. It is a figure of speech inherited and modified over time. The exact origins of many idioms are not known, but are thought to be rooted in creative storytelling or real events. This has led to idioms being called colloquial metaphors.
Another difference between an idiom and a phrase is that there are only two broad types of idiom. The first is the opaque idiom. These are where the idiom has no relation to the meaning, like with ‘kick the bucket.’ The second is a transparent idiom. These have a clearer meaning such as ‘spill the beans.’
There are, however, five types of phrase. Noun phrases are a simple phrase where the head is a noun. These nouns can be replaced with pronouns. They can also be modified using determiners, adjectives and complements. A verb phrase is much the same as a noun phrase, but where the verb is the head.
The prepositional phrases place a preposition before the noun. This creates phrases such as ‘in love’ and ‘on the roof.’ They are different to collocations because a collocation is the combination of two nouns, adjectives or verbs without any prepositions or articles. Some languages such as Japanese use postpositional phrases where the particle is tacked on to the end of the word.
Appositive phrases are those that fall between commas within a sentence. An appositive phrase involves a noun or pronoun phrase used within another noun phrase in order to modify it. An absolute phrase modifies a whole sentence, but has no syntax link to the main clause of the sentence. Absolute phrases usually come at the end of a sentence.