A meter has been officially defined to be the distance traveled by light in a vacuum in 1/299,792,458 second. All other length and distance measurements in the metric system derive from the meter (eg. km= 1000m, 1m= 1000mm).
Imperial / American measurements
These measurements have a less logical progression. A yard can be defined as the length of a pendulum that causes its arc to swing in exactly 1 second. The Nautical mile is the distance across 1' (1/60 of a degree) around the earths surface.