1Of the order of a millimetre in length or thickness; relating to or employing electromagnetic radiation having a wavelength of 1 to 10 millimetres; measured or expressed in millimetres.
2 rare Generally. As small as a millimetre (in figurative contexts); (of a distinction) very fine; (of accuracy) extreme.
Late 19th century; earliest use found in Appletons' Journal. From millimetre + -ic, after metric.