Definition
SI unit
An SI unit is a measurement unit that belongs to the International System of Units.
What it means
SI is a shared measurement language used around the world. Its seven base units are the second, metre, kilogram, ampere, kelvin, mole, and candela. Other SI units are built from these, and prefixes such as milli- or kilo- make decimal parts or multiples.
Remember it
SI units are an agreed measurement toolkit shared across countries and sciences.
Example
A race time of 12 s uses the second, an SI base unit. A length of 250 mm also uses an SI unit because millimetre is formed from metre with the SI prefix milli-.
Watch for
The minute and hour are accepted for use with SI, but they are non-SI units rather than SI units themselves.
Common mix-up
“Metric” and “SI” are often used together, but SI is the formally defined system and not every familiar metric-era unit belongs to it.
Exact meaning
An SI unit is a unit defined within the International System of Units, including its seven base units and units derived coherently from them.