A force that may be perceived to exist by an observer whose frame of reference is not inertial. Suppose, for example, that a frame of reference with origin O is rotating relative to an inertial frame of reference with the same origin. A particle P subject to a certain total force satisfies Newton’s second law of motion, relative to the inertial frame of reference. To the observer in the rotating frame, the particle appears to satisfy an equation of motion that is Newton’s second law of motion with additional terms. The observer may suppose that these terms are explained by certain fictitious forces.

In the special case where the rotating frame of reference has a constant angular velocity and the particle is moving in a plane perpendicular to the angular velocity, one fictitious force is in the direction along OP and is called the centrifugal force. This is the force outwards that is believed to exist by a rider on a roundabout. The second fictitious force is perpendicular to the path of P as seen by the observer in the rotating frame of reference and is called the Coriolis force.

In general, if S denotes the rotating frame and ω‎ denotes the angular velocity, then

where F is the total force acting, a denotes the perceived acceleration in S, r denotes position vector, and the subscript S denotes measurement in the non-inertial frame S. The three terms that appear to distort Newton’s second law are, in order, the Euler force (resulting from any angular acceleration), the Coriolis force, and the centrifugal force.