Frame of Reference: A frame in which an observer is situated and makes his observations is known as his ‘Frame of reference’.

Or

A frame associated with the observer with co-ordinate system and  a clock attached to it is known as Frame of Reference

Frame of reference are of two types :  (i) Inertial frame of reference  (ii) Non-inertial frame of reference.

Inertial frame of reference :  A frame of reference which is at rest or which is moving with a uniform velocity along a straight line is called an inertial frame of reference.

 In inertial frame of reference Newton’s laws of motion holds good.

Example – a man moving with a uniform velocity.

 Non inertial frame of reference : Accelerated frame of references are called non-inertial frame of reference.

 Newton’s laws of motion are not applicable in non-inertial frame of reference.

IF body moving in a circle is considered as frame of reference then it is non-inertial.