km/s vs kilometres per second

km/s and kilometres per second both are nouns.

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

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

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

Other synonyms of km/s include kilometers per second.

km/s (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).