grin vs facial expression

grin and facial expression both are nouns.

Word	Noun	Adjective	Verb	Adverb
grin	Yes	No	Yes	No
facial expression	Yes	No	No	No

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

grin: a facial expression characterized by turning up the corners of the mouth; usually shows pleasure or amusement
facial expression: a gesture executed with the facial muscles

Other hypernyms of grin include facial gesture.

grin (noun)	facial expression (noun)
a facial expression characterized by turning up the corners of the mouth; usually shows pleasure or amusement	the feelings expressed on a person's face
	a gesture executed with the facial muscles

grin (verb)	facial expression (verb)
to draw back the lips and reveal the teeth, in a smile, grimace, or snarl