means A metric unit of length equal to one-thousandth of a meter, used when precision down to the tiny matters.

from Built entirely from Latin-flavored scientific parts by the metric system: 'milli-' from Latin 'mille' meaning 'thousand' (the same root behind 'mile' and 'million'), bolted onto 'meter,' which comes through French 'mètre' from Greek 'metron,' 'a measure.' The metric system itself was forged in the aftermath of the French Revolution, when reformers wanted measurement built on reason rather than the length of some long-dead king's forearm — so the millimeter is, quite literally, a thousandth of a Greek measuring-word wearing a Latin prefix.