lactose

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A white crystalline disaccharide, C₁₂H₂₂O₁₁, found in milk, that may be hydrolyzed to yield glucose and galactose. Refined lactose obtained from whey is used in infant foods, bakery products, confections, and pharmaceuticals as a diluent and excipient.

from The Century Dictionary.

• noun Sugar of milk, C₁₂H₂₂O₁₁, obtained by evaporating whey, filtering through animal charcoal, and crystallizing.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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-(β-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 β-galactosidase.
• noun (Chem.) See Galactose.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biochemistry The disaccharide sugar of milk and dairy products, C12H22O11, (a product of glucose and galactose) used as a food and in medicinal compounds.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a sugar comprising one glucose molecule linked to a galactose molecule; occurs only in milk