tumulus vs burial mound

tumulus and burial mound both are nouns.

Word	Noun	Adjective	Verb	Adverb
tumulus	Yes	No	No	No
burial mound	Yes	No	No	No

As nouns, tumulus and burial mound are synonyms defined as:

tumulus and burial mound: (archeology) a heap of earth placed over prehistoric tombs

Other synonyms of tumulus include barrow, grave mound.

tumulus (noun)	burial mound (noun)
(archeology) a heap of earth placed over prehistoric tombs	(archeology) a heap of earth placed over prehistoric tombs