bike vs dirt bike

bike and dirt bike both are nouns.

Word	Noun	Adjective	Verb	Adverb
bike	Yes	No	Yes	No
dirt bike	Yes	No	No	No

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

bike: a motor vehicle with two wheels and a strong frame
dirt bike: a lightweight motorcycle equipped with rugged tires and suspension; an off-road motorcycle designed for riding cross country or over unpaved ground

Other hyponyms of bike include minibike, motorbike, scrambler, trail bike.

bike (noun)	dirt bike (noun)
a wheeled vehicle that has two wheels and is moved by foot pedals	a lightweight motorcycle equipped with rugged tires and suspension; an off-road motorcycle designed for riding cross country or over unpaved ground
a motor vehicle with two wheels and a strong frame

bike (verb)	dirt bike (verb)
ride a bicycle