downtime vs period of time

downtime and period of time both are nouns.

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

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

downtime: a period of time when something (as a machine or factory) is not operating (especially as a result of malfunctions)
period of time: an amount of time

Other hypernyms of downtime include period, time period.

downtime (noun)	period of time (noun)
a period of time when something (as a machine or factory) is not operating (especially as a result of malfunctions)	an amount of time