reign vs period of time

reign and period of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
reign	Yes	No	Yes	No
period of time	Yes	No	No	No

As nouns, period of time is a hypernym of reign; that is, period of time is a word with a broader meaning than reign:

Other hypernyms of reign include period, time period.

reign (noun)	period of time (noun)
royal authority; the dominion of a monarch	an amount of time
the period during which a monarch is sovereign
a period during which something or somebody is dominant or powerful

reign (verb)	period of time (verb)
have sovereign power
be larger in number, quantity, power, status or importance