Definitions of rider:

• noun:   a clause that is appended to a legislative bill
• noun:   a traveler who actively rides an animal (as a horse or camel)
• noun:   a traveler who actively rides a vehicle (as a bicycle or motorcycle)
• noun:   a traveler riding in a vehicle (a boat or bus or car or plane or train etc) who is not operating it
• name:  A surname (common: 1 in 20000 families; popularity rank in the U.S.: #2410)