sternum vs breastbone

sternum and breastbone both are nouns.

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

As nouns, sternum and breastbone are synonyms defined as:

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

sternum (noun)	breastbone (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