day vs unit of time

day and unit of time both are nouns.

Word	Noun	Adjective	Verb	Adverb
day	Yes	No	No	No
unit of time	Yes	No	No	No

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

Other hypernyms of day include time unit.

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

day: the time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day
unit of time: a unit for measuring time periods

Other hypernyms of day include time unit, sidereal time.

day (noun)	unit of time (noun)
a period of opportunity	a unit for measuring time periods
time for Earth to make a complete rotation on its axis
a day assigned to a particular purpose or observance
the time after sunrise and before sunset while it is light outside
a point or period in time
the recurring hours when you are not sleeping (especially those when you are working)
an era of existence or influence
the time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day
the period of time taken by a particular planet (e.g. Mars) to make a complete rotation on its axis