what is fictitious force??

A fictitious force, also called a phantom force, pseudo force, d''Alembert force  or inertial force,  is an apparent force that acts on all masses in a non-inertial frame of reference, such as a rotating reference frame.
The force F does not arise from any physical interaction but rather from the acceleration a of the non-inertial reference frame itself. As stated by Iro: Such an additional force due to nonuniform relative motion of two reference frames is called a pseudo-force.
— H Iro in A Modern Approach to Classical Mechanics p. 180
According to Newton''s second law in the form F = ma, fictitious forces always are proportional to the acceleration a.
A fictitious force arises when a frame of reference is accelerating compared to a non-accelerating frame. As a frame can accelerate in any arbitrary way, so can fictitious forces be as arbitrary (but only in direct response to the acceleration of the frame). However, four fictitious forces are defined for frames accelerated in commonly occurring ways: one caused by any relative acceleration of the origin in a straight line (rectilinear acceleration),  two caused by any rotation (centrifugal force and Coriolis force) and a fourth, called the Euler force, caused by a variable rate of rotation, should that occur.

A fictitious force, also called a phantom force, pseudo force d''Alembert force or inertial force is an apparent force that acts on all masses in a non-inertial frame of reference, such as a rotating reference frame.

a fictious force or pseudo force is an imaginary force that acts on a body in non inertial frame. it is opposite in direction of accelaration of non inertial frame.