blink of an eye vs split second

blink of an eye and split second both are nouns.

Word	Noun	Adjective	Verb	Adverb
blink of an eye	Yes	No	No	No
split second	Yes	No	No	No

As nouns, blink of an eye and split second are synonyms defined as:

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

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

blink of an eye (noun)	split second (noun)
a very short time (as the time it takes the eye to blink or the heart to beat)	a very short time (as the time it takes the eye to blink or the heart to beat)