A prefix meaning one-thousandth (1/1,000 or 10-3)
Example: a millimeter is 1/1,000th of a meter (in other words 1,000 millimeters make 1 meter)
Symbol is m (as a prefix, otherwise "m" means meter)
Examples:
12 ms = 12 milliseconds = 12 thousandths of a second (0.012 s)
950 mm = 950 millimeters = 950 thousandths of a meter (0.95 m)
See: Centi-