ISO 31-5 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to electricity and magnetism. It is superseded by ISO 80000-6.
Some of its definitions are below, with values taken from NIST values of the constants:
NameSymbolDefinitionValueSpeed of light in vacuumc0299 792 458 m s−1299 792 458 m s−1Magnetic constantμ04π × 10−7 N A−212.566 370 614... × 10−7 N A−2Electric constantε08.854 187 817... × 10−12 F m−1Characteristic impedance of vacuumZ0μ0 c0376.730 313 461...Ω References