split second vs blink of an eye

split second and blink of an eye both are nouns.

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

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

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

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

split second (noun)	blink of an eye (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)