Biochemistry. a disaccharide, C 12 H 22 O 11 , present in milk, that upon hydrolysis yields glucose and galactose.
a white, crystalline, sweet, water-soluble commercial form of this compound, obtained from whey and used in infant feedings, in confections and other foods, in bacteriological media, and in pharmacology as a diluent and excipient.

1855–60; lact- + -ose2 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
lactose (ˈlæktəʊs, -təʊz)
Also called: milk sugar a white crystalline disaccharide occurring in milk and used in the manufacture of pharmaceuticals and baby foods. Formula: C12H22O11

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

sugar from milk, 1858, from Fr., coined by Fr. chemist Marcelin-Pierre-Eugène Berthelot (1827-1907) from L. lac "milk" (see lactation) + suffix -ose.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

lactose lac·tose (lāk'tōs')

  1. A disaccharide in milk that hydrolyzes to yield glucose and galactose. Also called milk sugar.

  2. A white crystalline substance obtained from whey and used in infant foods and in pharmaceuticals as a diluent and excipient. Also called milk sugar.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
lactose   (lāk'tōs')  Pronunciation Key 
  1. A white crystalline disaccharide consisting of a glucose and a galactose molecule, found in milk and used in the manufacture of various other foods. Chemical formula: C12H22O11.

  2. The inability to digest lactose properly is called lactose intolerance. It is caused by a deficiency of the enzyme lactase and marked by abdominal cramping and other symptoms after ingesting lactose.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Coconut milk can also serve as the base of non-dairy ice creams for those
  lactose intolerant ice cream lovers.
Lactose is extracted by removing proteins and then dissolved to form glucose
  and galactose.
Also, the two definitive bacteria in yogurt eat lactose as they ferment milk,
  making yogurt a safe bet for the lactose-intolerant.
The researchers isolated the arsenic-switch gene and attached it to the first
  gene involved in the breakdown of lactose.
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