split second vs heartbeat

split second and heartbeat both are nouns.

Word	Noun	Adjective	Verb	Adverb
split second	Yes	No	No	No
heartbeat	Yes	No	No	No

As nouns, split second and heartbeat are synonyms defined as:

split second and heartbeat: a very short time (as the time it takes the eye to blink or the heart to beat)

Other synonyms of split second include blink of an eye, flash, instant, jiffy, New York minute, trice, twinkling, wink.

split second (noun)	heartbeat (noun)
a very short time (as the time it takes the eye to blink or the heart to beat)	an animating or vital unifying force
	the rhythmic contraction and expansion of the arteries with each beat of the heart
	a very short time (as the time it takes the eye to blink or the heart to beat)