The Collaborative International Dictionary of English v.0.48:

Galactose \Ga*lac"tose\, n. (Chem.)
   A white, crystalline sugar, C6H12O6, isomeric with
   dextrose, obtained by the decomposition of milk sugar, and
   also from certain gums. When oxidized it forms mucic acid.
   Called also lactose (though it is not lactose proper).
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

lactose \lac"tose`\ (l[a^]k"t[=o]s`), n.
   1. (Physiol. Chem.) The main sugar present in milk, called
      also sugar of milk or milk sugar. When isolated pure
      it is obtained crystalline; it is separable from the whey
      by evaporation and crystallization. It is a disaccharide
      with the formula C12H22O11, being chemically
      4-([beta]-D-galactosido)-D-glucose. It has a slightly
      sweet taste, is dextrorotary, and is much less soluble in
      water than either cane sugar or glucose. Formerly called
      lactin. When hydrolyzed it yields glucose and galactose.
      In cells it may be hydrolyzed by the enzyme
      [beta]-galactosidase.
      [1913 Webster +PJC]

   2. (Chem.) See Galactose.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lactose
    n 1: a sugar comprising one glucose molecule linked to a
         galactose molecule; occurs only in milk; "cow's milk
         contains about 4.7% lactose" [syn: lactose, milk sugar]