1 a person who rides (esp. a horse).
2 a an additional clause amending or supplementing a document. b Brit. Parl. an addition or amendment to a bill at its third reading. c a corollary. d Brit. a recommendation etc. added to a judicial verdict.
3 Math. a problem arising as a corollary of a theorem etc.
4 a piece in a machine etc. that surmounts or bridges or works on or over others.
5 (in pl.) an additional set of timbers or iron plates strengthening a ship's frame.
Derivatives:
riderless adj.
Etymology: OE ridere (as RIDE)
Useful english dictionary. 2012.