A pseudo force (also called a fictitious force or inertial force) is an apparent force that is observed only when viewing motion from a non-inertial (accelerating or rotating) reference frame. It does not arise from a physical interaction but is introduced to apply Newton's laws in an accelerating frame.
Definition
A pseudo force acts on an object when the observer is in an accelerating frame of reference. Its magnitude is
F=ma
where:
m = mass of the object
a= acceleration of the reference frame
The negative sign indicates the pseudo force acts opposite to the acceleration of the frame.
Example
Suppose you are standing in a lift that suddenly accelerates upward:
You feel heavier then your actual weight.
Similarly, if lift is moving downward, then you feel lighter .