In classical mechanics, the Euler force is the fictitious tangential force that is felt in reaction to any angular acceleration. That reactive acceleration is the Euler acceleration (named for Leonhard Euler), also known as azimuthal acceleration or transverse acceleration. In other words, it is an acceleration that appears when a non-uniformly rotating reference frame is used for analysis of motion and there is variation in the angular velocity of the reference frame's axes. This article is restricted to a frame of reference that rotates about a fixed axis.