ski vs roller ski

ski and roller ski both are nouns.

Word	Noun	Adjective	Verb	Adverb
ski	Yes	No	Yes	No
roller ski	Yes	No	No	No

As nouns, roller ski is a hyponym of ski; that is, roller ski is a word with a more specific, narrower meaning than ski:

ski: narrow wood or metal or plastic runners used in pairs for gliding over snow
roller ski: A ski with wheels for use on dry pavement, in the absence of snow.

Other hyponyms of ski include water ski, touring ski.

ski (noun)	roller ski (noun)
narrow wood or metal or plastic runners used in pairs for gliding over snow	A ski with wheels for use on dry pavement, in the absence of snow.

ski (verb)	roller ski (verb)
move along on skis