From 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).