In physics, centrifugal force (from Latin centrum "center" and fugere "to flee") is a fictitious force that appears when describing physics in a rotating reference frame; it acts on anything with mass considered in such a frame. Centrifugal force is fictitious because although it may feel to a person like a certain force is being exerted on them, someone outside the scene will see something different.
Example: If John is in a car that takes a sharp right turn, he will feel as though he is being pushed to his left. This is an imaginary force, called a centrifugal force, or a "running away from the center" force. John feels it because he is inside the car and is affected by it. However, if John's friend, Andy, is on the side of the road facing the front of John's car and watches John's car take a sharp right turn, Andy will see the car push John to the right with the car as it changes direction. This is a real force called centripetal force (or an "aiming towards the center" force) and acts towards the center of the circle of rotation.
In other words, John's body, traveling in a straight line before the car turned, wanted to keep moving in a straight line because it had momentum in that direction, but the car, which was exerting a force on John by turning, is pulling him to the right. John feels like his body is being pushed to the left as the car turns, but in fact, his body is being pulled to the right by the turning car.
This is a problem that appears in rotating situations. There is a difference between what appears to be happening if watched from the outside or if the event was seen from within a rotating object. These differing viewpoints are called frames of reference.
Despite the name, fictitious forces are experienced as very real by anyone whose immediate environment is a non-inertial frame. Even for observers in an inertial frame, fictitious forces provide a natural way to discuss dynamics within rotating environments such as planets, centrifuges, carousels, turning cars, and spinning buckets.