Longman Dictionary of Contemporary English
The Collaborative International Dictionary
Milli- \Mil"li-\ (m[i^]l"l[i^]-). [From L. mille a thousand.] (Metric System, Elec., Mech., etc.) A prefix denoting a thousandth part of; as, millimeter, milligram, milliamp[`e]re.
Douglas Harper's Etymology Dictionary
word-forming element meaning "thousandth part of a metric unit," from comb. form of Latin mille "thousand" (see million).
Wiktionary
pre. 1 In the International System of Units and other metric systems of units, one-thousandth of the unit to which it is prefixed; that is, multiplying the unit to which it is attached by 10-3. Symbol: m. 2 thousand; one-thousandth.
Wikipedia
Milli- (symbol m) is a unit prefix in the metric system denoting a factor of one thousandth (10). Proposed in 1793 and adopted in 1795, the prefix comes from the Latin , meaning "one thousand" (the Latin plural is ). Since 1960, the prefix is part of the International System of Units (SI).
Example:
- Snow is crystalline water and is about 1 millimetre in diameter.