A phrase is “a small group of words standing together as a conceptual unit”, while an idiom is “a group of words established by usage as having a meaning not deducible from those of the individual words”. So, the difference is that an idiom as an established meaning not directly linked to the individual words. Any idiom is a phrase.As an example, “raining cats and dogs” is both an idiom and a phrase. “A herd of cats” is a phrase but not an idiom.expression has about the same meaning as phrase, except it is usually used of a phrase which is in common use.So an idiom is a certain sort of expression, which in turn is a subset of phraseidiom > expressionphrase.
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Phrase is a group of words that are used together in a fixed expression . Examples : take off , do away with etc.

Idioms are expressions whose meaning is different, attractive and unique . It can be one word or group of words .Examples : catch one's eye , be glad to see the back of etc.