Millimeter \Mil"li*me`ter\, Millimetre \Mil"li*me`tre\, n. [F. millim[`e]tre; milli- milli- + m[`e]tre. See 3d Meter.]

A lineal measure in the metric system, containing the thousandth part of a meter; equal to .03937 of an inch. See 3d Meter.

Source: Webster's Revised Unabridged Dictionary (1913)