mm Hg vs pressure unit

mm Hg and pressure unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
mm Hg	Yes	No	No	No
pressure unit	Yes	No	No	No

As nouns, pressure unit is a hypernym of mm Hg; that is, pressure unit is a word with a broader meaning than mm Hg:

mm Hg: a unit of pressure equal to 0.001316 atmosphere; named after Torricelli
pressure unit: a unit measuring force per unit area

mm Hg (noun)	pressure unit (noun)
a unit of pressure equal to 0.001316 atmosphere; named after Torricelli	a unit measuring force per unit area