expression vs diction

expression and diction both are nouns.

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

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

expression (noun)	diction (noun)
the act of forcing something out by squeezing or pressing	the manner in which something is expressed in words
the feelings expressed on a person's face	the articulation of speech regarded from the point of view of its intelligibility to the audience
a word or phrase that particular people use in particular situations
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