bicyclist vs biker

bicyclist and biker both are nouns.

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

As nouns, bicyclist and biker are synonyms defined as:

Other synonyms of bicyclist include bicycler, cyclist, wheeler.

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