expression vs effusion

expression and effusion both are nouns.

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

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

Other hyponyms of expression include lamentation, mourning, act, blowup, ebullition, gush, outburst.

expression (noun)	effusion (noun)
the act of forcing something out by squeezing or pressing	flow under pressure
the feelings expressed on a person's face	an unrestrained expression of emotion
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