fortnight vs period of time

fortnight and period of time both are nouns.

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

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

Other hypernyms of fortnight include period, time period.

fortnight (noun)	period of time (noun)
a period of fourteen consecutive days	an amount of time