rock elm vs American elm

rock elm and American elm both are nouns.

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

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

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

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

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