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 It is superseded by ISO 80000-6.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:

Name	Symbol	Definition	Value
Speed of light in vacuum	c0	299 792 458 m s−1	299 792 458 m s−1
Magnetic constant	μ0	4π × 10−7 N A−2	12.566 370 614... x 10−7 N A−2
Electric constant	ε0	${\displaystyle {\begin{matrix}{\frac {1}{\mu _{0}{c_{0}}^{2}}}\end{matrix}}}$	8.854 187 817... x 10−12 F m−1
Characteristic impedance of vacuum	Z0	μ0 c0	376.730 313 461...Ω