/ ˈraɪdə(r); NAmE / noun
1.
a person who rides a horse, bicycle or motorcycle :
Three riders (= people riding horses) were approaching.
horses and their riders
She's an experienced rider.
a motorcycle dispatch rider
—picture at showjumping
2.
rider (to sth) an extra piece of information that is added to an official document
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WORD ORIGIN
late Old English rīdere mounted warrior, knight (see ride , -er ).