lactometer|lækˈtɒmɪtə(r)| [f. lacto- + -meter.] An instrument for gauging the purity of milk. 1817Blackw. Mag. II. 219 A Lactometer, for ascertaining the comparative value of each cow's milk in a dairy.1872Echo 8 Oct. 5 Milk which was proved by the lactometer to be more than half water.