twenty-four hours vs twenty-four hour period

twenty-four hours and twenty-four hour period both are nouns.

Word	Noun	Adjective	Verb	Adverb
twenty-four hours	Yes	No	No	No
twenty-four hour period	Yes	No	No	No

As nouns, twenty-four hours and twenty-four hour period are synonyms defined as:

twenty-four hours and twenty-four hour period: time for Earth to make a complete rotation on its axis

Other synonyms of twenty-four hours include 24-hour interval, day, mean solar day, solar day.

twenty-four hours (noun)	twenty-four hour period (noun)
time for Earth to make a complete rotation on its axis	time for Earth to make a complete rotation on its axis