immovable vs mortmain

immovable and mortmain both are nouns.

Word	Noun	Adjective	Verb	Adverb
immovable	Yes	Yes	No	No
mortmain	Yes	No	No	No

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

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

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

immovable (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)

immovable (adjective)	mortmain (adjective)
not able or intended to be moved