a blade, plate, sail, etc., in the wheel of a windmill, to be moved by the air.
any of a number of blades or plates attached radially to a rotating drum or cylinder, as in a turbine or pump, that move or are moved by a fluid, as steam, water, hot gases, or air.
Aerospace. a. any fixed or movable plane surface on the outside of a rocket providing directional control while the rocket is within the atmosphere. b. a similar plane surface located in the exhaust jet of a reaction engine, providing directional control while the engine is firing.
Ornithology. the web of a feather.
Navigation, Surveying. either of two fixed projections for sighting an alidade or the like.
It must be admitted that he was a vane, turning on a pivot finer than those on which statesmen have generally been made to work.
-- Anthony Trollope, The Life of Cicero, 1880
Spewing out “chaff,” that reanimated dead metaphor in British English for useless verbiage or humbug, the grinding nonsense of the Office serves merely as a weather vane for the clichéd winds of change.
-- Garrett Stewart, Edited by John O. Jordan, “Dickens and Language,” The Cambridge Companion to Charles Dickens, 2001
Vane is a variant of the word fane, meaning "flag; banner." It entered English in the mid-1400s.