kilometres per second vs km/s

kilometres per second and km/s both are nouns.

Word	Noun	Adjective	Verb	Adverb
kilometres per second	Yes	No	No	No
km/s	Yes	No	No	No

As nouns, kilometres per second and km/s are synonyms defined as:

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

Other synonyms of kilometres per second include kilometers per second.

kilometres per second (noun)	km/s (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).