What fictitious force appears outward in a rotating reference frame?

The fictitious force that appears outward in a rotating reference frame is the centrifugal force.

It is called fictitious, or inertial, because it does not arise from a physical interaction such as gravity or contact. Instead, it appears when motion is described from a frame that is rotating. Its magnitude is mω²r, where m is mass, ω is angular speed, and r is perpendicular distance from the rotation axis.

People on a spinning carousel feel pushed outward, so centrifugal force is a useful description from the carousel's rotating frame. In an inertial frame, however, the object naturally tends to travel in a straight line while the carousel or seat pushes it inward. That inward real force is centripetal force.

Centrifugal force should not be confused with the Coriolis force. Centrifugal force points outward and exists for an object at rest in the rotating frame; Coriolis force affects objects moving relative to that frame and deflects their apparent path.