pacemaker vs pacer

pacemaker and pacer both are nouns.

Word	Noun	Adjective	Verb	Adverb
pacemaker	Yes	No	No	No
pacer	Yes	No	No	No

As nouns, pacemaker and pacer are synonyms defined as:

Other synonyms of pacemaker include pacesetter.

pacemaker (noun)	pacer (noun)
a horse used to set the pace in racing	a horse trained to a special gait in which both feet on one side leave the ground together
an implanted electronic device that takes over the function of the natural cardiac pacemaker	a horse used to set the pace in racing
a specialized bit of heart tissue that controls the heartbeat
a leading instance in its field