contusion vs petechia

contusion and petechia both are nouns.

Word	Noun	Adjective	Verb	Adverb
contusion	Yes	No	No	No
petechia	Yes	No	No	No

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

contusion: an injury that doesn't break the skin but results in some discoloration
petechia: a minute red or purple spot on the surface of the skin as the result of tiny hemorrhages of blood vessels in the skin (as in typhoid fever)

Other hyponyms of contusion include ecchymosis, black eye, mouse, shiner.

contusion (noun)	petechia (noun)
the action of bruising	a minute red or purple spot on the surface of the skin as the result of tiny hemorrhages of blood vessels in the skin (as in typhoid fever)
an injury that doesn't break the skin but results in some discoloration