Dictionary.com
definitions

milli-

a combining form meaning “thousand” (millipede): in the metric system, used in the names of units equal to one thousandth of the given base unit (millimeter).

Origin of milli-

< French < Latin, combining form of mille thousand
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for milli-

milli-

prefix

denoting 10 –3millimetre Symbol: m

Word Origin for milli-

from French, from Latin mille thousand, this meaning being maintained in words borrowed from Latin (millipede)
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 milli-

word-forming element meaning "thousandth part of a metric unit," from comb. form of Latin mille "thousand" (see million).

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

milli-

pref.

One thousandth (10-3):millisecond.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
milli- in Science

milli-

A prefix that means “one thousandth,” as in millimeter, one thousandth of a meter.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.