root celery vs celeriac

root celery and celeriac both are nouns.

Word	Noun	Adjective	Verb	Adverb
root celery	Yes	No	No	No
celeriac	Yes	No	No	No

As nouns, root celery and celeriac are synonyms defined as:

Other synonyms of root celery include Apium graveolens rapaceum, celery root, knob celery, turnip-rooted celery.

root celery (noun)	celeriac (noun)
grown for its thickened edible aromatic root	thickened edible aromatic root of a variety of celery plant
	grown for its thickened edible aromatic root