rider

noun

1. a person who rides a horse or other animal, a bicycle, etc.
2. something that rides.
3. an additional clause, usually unrelated to the main body, attached to a legislative bill in passing it.
4. an addition or amendment to a document, testament, etc.
5. any object or device that straddles, is mounted upon, or is attached to something else.
6. a rail or stake used to brace the corners in a snake fence.
7. Shipbuilding. any of various members following and reinforcing primary framing members, especially a plate or timber running along the top of a keel.
8. Numismatics.
   1. a former gold coin of Scotland, first issued by James III in 1475, whose obverse bears an equestrian figure of the king.
   2. any of several gold or silver coins of the Netherlands bearing the figure of a horseman.

noun

1. a person or thing that rides, esp a person who rides a horse, a bicycle, or a motorcycle
2. an additional clause, amendment, or stipulation added to a legal or other document, esp (in Britain) a legislative bill at its third reading
3. British a statement made by a jury in addition to its verdict, such as a recommendation for mercy
4. any of various objects or devices resting on, surmounting, or strengthening something else
5. a small weight that can be slid along one arm of a chemical balance to make fine adjustments during weighing
6. geology a thin seam, esp of coal or mineral ore, overlying a thicker seam

n.

“one who rides,” Old English ridere “rider, trooper, knight, mounted warrior,” agent noun from ride (v.). Meaning “clause tacked on to a document after first draft” is from 1660s. Related: Riderless.

A provision, usually controversial and unlikely to pass on its own merits, that is attached to a popular bill in the hopes that it will “ride” to passage on the back of the popular bill.