idiom vs expression

idiom and expression both are nouns.

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

As nouns, expression is a hypernym of idiom; that is, expression is a word with a broader meaning than idiom:

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

Other hypernyms of idiom include locution, saying.

As nouns, expression is a hypernym of idiom; that is, expression is a word with a broader meaning than idiom:

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

Other hypernyms of idiom include formulation.

idiom (noun)	expression (noun)
the style of a particular artist or school or movement	the act of forcing something out by squeezing or pressing
the usage or vocabulary that is characteristic of a specific group of people	the feelings expressed on a person's face
an expression whose meanings cannot be inferred from the meanings of the words that make it up	a word or phrase that particular people use in particular situations
a manner of speaking that is natural to native speakers of a language	the style of expressing yourself
	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