lactose|ˈlæktəʊs| [f. L. lact-, lac + -ose2. Cf. F. lactose.] A saccharine substance present in milk, commonly called sugar of milk. 1858Fownes' Chem. (ed. 7) 410 Sugar of milk; lactin; lactose.1869Roscoe Elem.Chem. 369 Lactose, or milk sugar, occurs only in the milk of mammalia.