米（m）
the fundamental unit of length in the international system of units (si). originally defined as 1/10,000,000 of the distance from the equator to the north pole, now defined by the speed of light.
practical reference points:
height of an average door handle: ~1 meter
one large step: ~1 meter
wavelength of red light: ~700 nm = 7×10次方 m
diameter of human hair: ~50-100 μm = 5×10-5 to 1×10-4 m
si unit: base unit for length in the metric system