"Systeme International", or SI Units, is a standardised system of measurement based on internationally agreed definitions.

It has 7 Base Units, from which all other units are derived:

Metre (m) - Length

Kilogram (Kg) - Mass

Second (s) - Time

Kelvin (°K) - temperature

Ampere (A) - Current

Candela (Cd) - Luminous Intensity

Mole (mol) - Amount of a substance

All the base units have standard definition, for example, the metre is the distance travelled by light in a vacuum in seconds.

Other units are derived from the base units. For example:

Velocity (ms^{-1})

Acceleration (ms^{-2})

Density (Kgm^{-2})

Some of the derived units also have their own special unit names. For example:

Coulomb (C) - (As) Charge

Pascal (Pa) - (Kgm^{-1}s^{-2}) Pressure

Ohms (Ω) - (Kgm^{2}s^{-3}A^{-2}) Resistence

Prefixes

In order to make working with large or small numbers more convenient, a system of prefixes is used, where the unit is multiplied by a certain power of ten:

Yotta (Y) - 10^{24}

Zetta (Z) - 10^{21}

Exa (E) - 10^{18}

Peta (P) - 10^{15}

Tera (T) - 10^{12}

Giga (G) - 10^{9}

Mega (M) - 10^{6}

Kilo (k) - 10^{3}

Milli (m) - 10^{-3}

Micro (μ) - 10^{-6}

Nano (n) - 10^{-9}

Pico (p) - 10^{-12}

Femto (f) - 10^{-15}

Atto (a) - 10^{-18}

Zepto (z) - 10^{-21}

Yocto (y) - 10^{-24}

In addition, these prefixes are also used:

Hecto (h) - 10^{2}

Deca (da) - 10^{1}

Deci (d) - 10-^{-1}

Centi (c) - 10^{-2}

For example, 0.005 A can be written as 5 mA, and 6900000000000000000000000 m can be written as 6.9 Ym.