kilometres per second vs kilometers per second

kilometres per second and kilometers per second both are nouns.

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

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

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

Other synonyms of kilometres per second include km/s.

kilometres per second (noun)	kilometers 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).