American elm vs rock elm

American elm and rock elm both are nouns.

Word	Noun	Adjective	Verb	Adverb
American elm	Yes	No	No	No
rock elm	Yes	No	No	No

As nouns, American elm and rock elm are synonyms defined as:

American elm and rock elm: large ornamental tree with graceful gradually spreading branches common in eastern North America

Other synonyms of American elm include Ulmus americana, water elm, white elm.

American elm (noun)	rock elm (noun)
large ornamental tree with graceful gradually spreading branches common in eastern North America	tall widely distributed elm of eastern North America
	large ornamental tree with graceful gradually spreading branches common in eastern North America