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lacto-

  1. a combining form meaning “milk,” used in the formation of compound words (lactometer); specialized in chemical terminology to mean “lactate,” or “lactic acid.”
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Also lacti-; especially before a vowel, lact-.

Origin of lacto-

< Latin lact- (stem of lac milk) + -o-
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for lacto-

lacto-

before a vowel lact-

combining form
  1. indicating milklactobacillus
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Word Origin

from Latin lact-, lac milk
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

Word Origin and History for lacto-

before vowels, lac-, word-forming element meaning "milk," from Latin lac (genitive lactis) "milk," from PIE root *glakt- (see lactation).

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Online Etymology Dictionary, © 2010 Douglas Harper
lacto- in Medicine

lacto-

pref.
  1. Milk:lactoglobulin.
  2. Lactose:lactase.
  3. Lactic acid:lactate.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.