The unit of degree refers to the basic unit of measuring spatial distance, which is formulated by human beings to standardize the length. Its international unit is "meter" (symbol "m"), and commonly used units are millimeter (mm), centimeter (cm), decimeter (dm), kilometer (km), meter (m), micron (micron) and nanometer (nm).
In the international system of units, the standard unit of length is "meter", which is represented by the symbol "m". 1960 11th International Conference on Metrology: "The length of meter is equal to 10 and 5d 1 energy levels of krypton -86 atom, which is 1650763.73 times of the radiation wavelength in vacuum".
Other length units are: light year, astronomical unit, meter (Pm), megameter (mm), kilometer (km), decimeter (dm), centimeter (cm), millimeter (Mm), silk rice (dmm), millet (cmm), micron (micron) and nanometer (nm).