kilometers per second vs kilometres per second

kilometers per second and kilometres per second both are nouns.

Word	Noun	Adjective	Verb	Adverb
kilometers per second	Yes	No	No	No
kilometres per second	Yes	No	No	No

As nouns, kilometers per second and kilometres per second are synonyms defined as:

kilometers per second and kilometres per second: the ratio of the distance traveled (in kilometers) to the time spent traveling (in seconds).

Other synonyms of kilometers per second include km/s.

kilometers per second (noun)	kilometres per second (noun)
the ratio of the distance traveled (in kilometers) to the time spent traveling (in seconds).	the ratio of the distance traveled (in kilometers) to the time spent traveling (in seconds).