sidereal day vs sidereal time

sidereal day and sidereal time both are nouns.

Word	Noun	Adjective	Verb	Adverb
sidereal day	Yes	No	No	No
sidereal time	Yes	No	No	No

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

sidereal day: the time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day
sidereal time: measured by the diurnal motion of stars

Other hypernyms of sidereal day include time unit, unit of time.

sidereal day (noun)	sidereal time (noun)
the time for one complete rotation of the earth relative to a particular star, about 4 minutes shorter than a mean solar day	measured by the diurnal motion of stars