split second vs mo

split second and mo both are nouns.

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

As nouns, mo is a hypernym of split second; that is, mo is a word with a broader meaning than split second:

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

Other hypernyms of split second include bit, minute, moment, second.

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