biker vs bicyclist

biker and bicyclist both are nouns.

Word	Noun	Adjective	Verb	Adverb
biker	Yes	No	No	No
bicyclist	Yes	No	No	No

As nouns, biker and bicyclist are synonyms defined as:

Other synonyms of biker include bicycler, cyclist, wheeler.

biker (noun)	bicyclist (noun)
a person who rides a bicycle	a person who rides a bicycle