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a combining form meaning “milk,” used in the formation of compound words (lactometer); specialized in chemical terminology to mean “lactate,” or “lactic acid.”.
Also, lacti-; especially before a vowel, lact-.
Origin of lacto-
< Latin lact- (stem of lac milk) + -o- Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for lacto-


combining form
indicating milk: lactobacillus
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
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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).

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

lacto- or lact- or lacti-

  1. Milk: lactoglobulin.

  2. Lactose: lactase.

  3. Lactic acid: lactate.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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