wind·mill [windmillwindmillswindmilledwindmilling] BrE [ˈwɪndmɪl] NAmE [ˈwɪndmɪl] noun 1. a building with machinery for ↑grinding grain into flour that is driven by the power of the wind turning long arms (called ↑sails) 2. a tall thin structure with parts that turn round, used to change the power of the wind into electricity 3. (BrE) (NAmE pin·wheel)a toy with curved plastic parts that form the shape of a flower which turns round on the end of a stick when you blow on it see tilt at windmills at ↑tiltv. See also: ↑pinwheel