space-time vs reference system

space-time and reference system both are nouns.

Word	Noun	Adjective	Verb	Adverb
space-time	Yes	No	No	No
reference system	Yes	No	No	No

As nouns, reference system is a hypernym of space-time; that is, reference system is a word with a broader meaning than space-time:

space-time: the four-dimensional coordinate system (3 dimensions of space and 1 of time) in which physical events are located
reference system: a system that uses coordinates to establish position

Other hypernyms of space-time include coordinate system, frame of reference, reference frame.

space-time (noun)	reference system (noun)
the four-dimensional coordinate system (3 dimensions of space and 1 of time) in which physical events are located	a system that uses coordinates to establish position