In addition of to the three fundamental SI base units, there are other four SI base units which are kelvin (K) for temperature, ampere (A) for electric current, candela (cd) for luminocity and mole (mol) for amount of substance.
Interesting thing you can notice in the definition of these SI units is that the SI unit of length which depends on the SI unit of time. And the SI unit of mass depends on the SI units of time and length.
For the recent redefinition of SI base units you may also check 2019 redefinition of the SI base units.
There are also other units other than the SI units used in specific regions of the world. We will only consider the SI units here. There are two types of SI units - one type is SI base units and another is SI derived units. The following table shows 7 SI base units and their corresponding physical quantities.
| SI Unit | Symbol | Physical Quantity |
|---|---|---|
| meter | m | length |
| second | s | time |
| kilogram | kg | mass |
| kelvin | K | temperature |
| ampere | A | current |
| candela | cd | luminous intensity |
| mole | mol | amount of substance |
The 7 SI base units in the above table can be used to derive all other SI units called SI derived units. For example the SI unit of speed is the SI derived unit using two base units meter and second, that is meters per second (m/s).
The two units radian for plane angle and steradian for solid angle are neither SI base units nor SI derived units. These two were expressed as SI supplementary units and don't hold the agreement to be SI base units yet. They are more likely considered as SI derived units.
