Measurement is a process of comparing a physical quantity with a standard quantity. The standard quantity used to compare a physical quantity for its measurement is called unit.
In the ancient times parts of human body were used for measurement but these led to conflicts and confusions because these were arbitrary, non uniform and led to results which were not reproducible.
The internationally accepted modern system of units used in science is known as SI units. In SI units there are 7 basic units. These are:
- metre for length
- kilogram for mass
- second for time
- Kelvin for temperature
- ampere for electric current
- candela for luminous intensity
- mole for amount of substances
The units obtained in terms of basic units are called derived units.
SI prefixes are used in cases where the quantity being measured is too large or too small as compared to the base unit of the physical quantity.