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lactose

American  
[lak-tohs] / ˈlæk toʊs /

noun

  1. Biochemistry. a disaccharide, C 12 H 22 O 11 , present in milk, that upon hydrolysis yields glucose and galactose.

  2. 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.

lactose British  
/ ˈlæktəʊs, -təʊz /

noun

  1. Also called: milk sugar.  a white crystalline disaccharide occurring in milk and used in the manufacture of pharmaceuticals and baby foods. Formula: C 12 H 22 O 11

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
lactose Scientific  
/ lăk′tōs′ /

  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: C 12 H 22 O 11 .

  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.

Etymology

Origin of lactose

First recorded in 1855–60; lact- + -ose 2

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

Shortly after Meadow Lane opened, customers posted online about undercooked chicken nuggets; another shopper, who was lactose intolerant, said that she’d bought chili topped with cheese that wasn’t on the list of ingredients.

From The Wall Street Journal • Feb. 10, 2026

“Being a backtalker is like being lactose intolerant,” she writes in “A Note From Kim.”

From Los Angeles Times • Jan. 30, 2026

It appears in milk and other dairy products when lactose breaks down under heat or enzymatic activity, including during the production of yogurt, cheese, and kefir.

From Science Daily • Jan. 13, 2026

Plant-based milks, despite existing for millennia, rose to global popularity amid the 21st century due to growing concerns of lactose intolerance and the environmental impacts of dairy.

From Salon • Aug. 30, 2025

Pardee, Jacob, and Monod published their monumental study on the lactose operon in 1959, six years after the Watson and Crick paper on the structure of DNA.

From "The Gene" by Siddhartha Mukherjee

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.