The term originate as he accurately stated - milling was the first major application. It then later expanded to include any device that harnesses the wind for mechanical power (including pumping). By contrast, a *wind turbine* is a device that converts wind power into *electricity*.
electricity by mechanical power. Indeed.
The wind'mill' in St. Margaret's Bay, Kent was built in 1929 to generate electricity for the adjoining house.
The same way e.g. a gas turbine in a fossil plant converts the energy of combustion gases into electricity, the way a steam turbine does the same with steam, etc. (Technically the turbine is only the energy-to-rotation conversion part, but today it's commonly used to describe systems as a whole)