ISO 31-5
Encyclopedia
ISO 31-5 is the part of international standard
International standard
International standards are standards developed by international standards organizations. International standards are available for consideration and use, worldwide...

 ISO 31
ISO 31
International Standard ISO 31 was the most widely respected style guide for the use of physical quantities and units of measurement, and formulas involving them, in scientific and educational documents worldwide...

 that defines names and symbols for quantities
Physical quantity
A physical quantity is a physical property of a phenomenon, body, or substance, that can be quantified by measurement.-Definition of a physical quantity:Formally, the International Vocabulary of Metrology, 3rd edition defines quantity as:...

 and units related to electricity and magnetism.

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
Electric constant
The physical constant ε0, commonly called the vacuum permittivity, permittivity of free space or electric constant is an ideal, physical constant, which is the value of the absolute dielectric permittivity of classical vacuum...

ε0 8.854 187 817... x 10−12 F m−1
Characteristic impedance of vacuum Z0 μ0 c0 376.730 313 461...Ω
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