a combining form meaning “milk,” used in the formation of compound words (lactometer); specialized in chemical terminology to mean “lactate,” or “lactic acid.”
Origin of lacto-
< Latin lact- (stem of lac milk) + -o-
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
before a vowel lact-
Word Origin for lacto-
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
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
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.