Although it's conventional to think about the "metric system" in terms of metres (= 1/40,000,000 of the circumference of the Earth at the equator), today the metre is better defined, and it's all called SI (systeme internationale d'unites). I tend to think of ANY system as "a metric system".
And so the nautical mile is 1 minute of longitude at the equator, so = 1/(360x60) of the circumference of the Earth at the equator.