"building fitted to grind grain," Old English mylen "a mill" (10c.), an early Germanic borrowing from Late Latin molina, molinum "mill" (source of French moulin, Spanish molino), originally fem. and neuter of molinus "pertaining to a mill," from Latin mola "mill, millstone," related to molere "to grind," from PIE *mele-, *mel- "to crush, grind," with derivatives referring to ground material and tools for grinding (cf. Greek myle "mill;" see mallet).
Also from Late Latin molina, directly or indirectly, are German Mühle, Old Saxon mulin, Old Norse mylna, Danish mølle, Old Church Slavonic mulinu. Broader sense of "grinding machine" is attested from 1550s. Other types of manufacturing machines driven by wind or water, whether for grinding or not, began to be called mills by early 15c. Sense of "building fitted with industrial machinery" is from c.1500.