real estate vs mortmain

real estate and mortmain both are nouns.

Word	Noun	Adjective	Verb	Adverb
real estate	Yes	No	No	No
mortmain	Yes	No	No	No

As nouns, mortmain is a hyponym of real estate; that is, mortmain is a word with a more specific, narrower meaning than real estate:

real estate: property consisting of houses and land
mortmain: real property held inalienably (as by an ecclesiastical corporation)

Other hyponyms of real estate include acres, demesne, estate, land, landed estate, land, dead hand.

real estate (noun)	mortmain (noun)
property consisting of houses and land	the oppressive influence of past events or decisions
	real property held inalienably (as by an ecclesiastical corporation)