rider in WordNet English dictionary

1. a traveler riding in a vehicle (a boat or bus or car or plane or train etc) who is not operating it
   passenger  rider 
   
2. a clause that is appended to a legislative bill
   rider 
   
3. a traveler who actively rides an animal (as a horse or camel)
   rider 
   
4. a traveler who actively rides a vehicle (as a bicycle or motorcycle)
   rider