decade vs period of time

decade and period of time both are nouns.

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

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

Other hypernyms of decade include period, time period.

decade (noun)	period of time (noun)
the cardinal number that is the sum of nine and one; the base of the decimal system	an amount of time
a period of 10 years