The SI unit of length is the METRE, which is a five-letter word. The METRE is a unit of length in the International System of Units (SI). It is the base unit of length in the SI and is defined as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. The METRE is also equal to 1 650 763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom.