What is an SI-unit?
An SI-unit is any unit of measurement defined within the International System of Units, which is the modern form of the metric system and the world's most widely used system of measurement.
could you list some of the most commonly used SI units and their definition?
Sure! Here are some of the most commonly used SI units and their definitions:

- **Meter (m)**: The SI unit of length.
- **Kilogram (kg)**: The SI unit of mass.
- **Second (s)**: The SI unit of time.
- **Kelvin (K)**: The Si unit of thermodynamic temperature.
- **Ampere (A)**: The SI unit of electric current.
- **Candela (cd)**: The SI unit of luminous intensity.
- **Mole (mol)**: The SI unit of amount of substance.