Unit:
It is a standard quantity of the same kind chosen in order to measure physical quantity.
Fundamental units:
The physical quantity which does not depend on any other quantity are called fundamental physical quantities.
the unit of fundamental quantity is called fundamental unit.
Or
The units selected for measuring mass, length, and time are called fundamental units.
Note: the fundamental units are not definable in terms of other quantities. for this reason, they are called base or fundamental units.
Derived units:
The units of physical quantities which can be expressed in terms of fundamental units.(mass, length and time) are called derived units.
(a) The unit of velocity is a derived unit
Velocity= displacement/time
= Length/time
= m/s
(b) unit of acceleration is a derived unit
Acceleration = velocity/time
= Displacement/time^2
= Length/time^2
= m/s^2