say it: RAHY-dur
rider is a noun meaning "a traveler who actively rides an animal (as a horse or camel)". It carries 4 recorded senses.
a traveler who actively rides an animal (as a horse or camel)
a clause that is appended to a legislative bill
a traveler who actively rides a vehicle (as a bicycle or motorcycle)
a traveler riding in a vehicle (a boat or bus or car or plane or train etc) who is not operating it
boat · bus · car
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for rider come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.