In the context of measurement, a unit is a fixed quantity of a physical amount which is defined and accepted by convention or/and rule and used as a standard for the measurement of that physical amount. Any other quantity of that physical amount is expressed as a multiplier of this 'unit'.
For example, length is a physical amount. 'Meter' is a unit of length equal to a certain predefined distance. When we say that such distance is '4 meters', it means that the said distance is 4 times of 1 meter.
Defining units, agreeing on them, making practical use of them has been a very important role since ancient times. It was common for units to have different systems in different places and times. But now a global standard system exists, which is called 'International System of Units (SI)'.
The weighing and measuring tables used in various fields are given below.