Entry 1 sense Webster, 1913 Milli- /(mĭl"lĭ-)/ · Mil·li- · IPA /ˈmɪlɪ/ 01 A prefix denoting a thousandth part of; as, millimeter, milligram, milliampère. 1. A prefix denoting a thousandth part of; as, millimeter, milligram, milliampère.(Metric System, Elec., Mech., etc.)