lactose

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A disaccharide, C₁₂H₂₂O₁₁, found in milk, that may be hydrolyzed to yield glucose and galactose.
2. n. A white crystalline substance obtained from whey and used in infant foods, bakery products, confections, and pharmaceuticals as a diluent and excipient. Also called milk sugar.

Century Dictionary and Cyclopedia

1. n. Sugar of milk, C₁₂H₂₂O₁₁, obtained by evaporating whey, filtering through animal charcoal, and crystallizing. It forms hard, white, semi-transparent trimetric crystals, which are less soluble than cane-or grape sugar, have a slightly sweet taste, and grate between the teeth. It is dextrogyrate, and ferments slowly with yeast, but readily undergoes the lactic fermentation. It is convertible into glucose and galactose by boiling with dilute sulphuric acid. It is used for food and in medicine, and is prepared as an article of commerce in Switzerland and Bavaria. Also called galactine lactine, and milk-sugar.

Wiktionary

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

GNU Webster's 1913

1. n. (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.
2. n. (Chem.) See Galactose.

WordNet 3.0

1. n. a sugar comprising one glucose molecule linked to a galactose molecule; occurs only in milk