breastbone vs sternum

breastbone and sternum both are nouns.

Word	Noun	Adjective	Verb	Adverb
breastbone	Yes	No	No	No
sternum	Yes	No	No	No

As nouns, breastbone and sternum are synonyms defined as:

breastbone and sternum: the flat bone that articulates with the clavicles and the first seven pairs of ribs

breastbone (noun)	sternum (noun)
the flat bone that articulates with the clavicles and the first seven pairs of ribs	the flat bone that articulates with the clavicles and the first seven pairs of ribs