expression vs idiom

expression and idiom both are nouns.

Word	Noun	Adjective	Verb	Adverb
expression	Yes	No	No	No
idiom	Yes	No	No	No

As nouns, idiom is a hyponym of expression; that is, idiom is a word with a more specific, narrower meaning than expression:

expression: a word or phrase that particular people use in particular situations
idiom: an expression whose meanings cannot be inferred from the meanings of the words that make it up

As nouns, idiom is a hyponym of expression; that is, idiom is a word with a more specific, narrower meaning than expression:

expression: the style of expressing yourself
idiom: a manner of speaking that is natural to native speakers of a language

expression (noun)	idiom (noun)
the act of forcing something out by squeezing or pressing	the style of a particular artist or school or movement
the feelings expressed on a person's face	the usage or vocabulary that is characteristic of a specific group of people
a word or phrase that particular people use in particular situations	an expression whose meanings cannot be inferred from the meanings of the words that make it up
the style of expressing yourself	a manner of speaking that is natural to native speakers of a language
expression without words
a group of symbols that make a mathematical statement
the communication (in speech or writing) of your beliefs or opinions
a group of words that form a constituent of a sentence and are considered as a single unit
(genetics) the process of expressing a gene