idiom vs treatment

idiom and treatment both are nouns.

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

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

idiom (noun)	treatment (noun)
the style of a particular artist or school or movement	care provided to improve a situation (especially medical procedures or applications that are intended to relieve illness or injury)
the usage or vocabulary that is characteristic of a specific group of people	the management of someone or something
an expression whose meanings cannot be inferred from the meanings of the words that make it up	(medicine) the act of caring for someone (as by medication or remedial training etc.)
a manner of speaking that is natural to native speakers of a language	a manner of dealing with something artistically
	an extended communication (often interactive) dealing with some particular topic