rud·der 
(rŭdər)
(rŭdər)n.
1.
a. A vertically hinged plate of metal, fiberglass, or wood mounted at the stern of a ship or boat for directing its course.
b. A similar structure at the tail of an aircraft, used for effecting horizontal changes in course.
2. A controlling agent or influence over direction; a guide.
[Middle English ruder, from Old English rōther, steering oar; see erə- in the Appendix of Indo-European roots.]