bit vs split second

bit and split second both are nouns.

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

As nouns, split second is a hyponym of bit; that is, split second is a word with a more specific, narrower meaning than bit:

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

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

bit (noun)	split second (noun)
the cutting part of a drill; usually pointed and threaded and is replaceable in a brace or bitstock or drill press	a very short time (as the time it takes the eye to blink or the heart to beat)
piece of metal held in horse's mouth by reins and used to control the horse while riding
the part of a key that enters a lock and lifts the tumblers
a short performance that is part of a longer program
a small fragment
an instance of some kind
a small amount of solid food; a mouthful
a small fragment of something broken off from the whole
a unit of measurement of information (from binary + digit); the amount of information in a system having two equiprobable states
a small piece or quantity of something
an indefinitely short time