1. An angle used to fix the position of a body, such as a planet, in an elliptical orbit. The true anomaly of a planet is the angle between the perihelion, the sun, and the planet in the direction of the planet's motion. The mean anomaly is the angle between the perihelion, the sun, and an imaginary planet having the same period as the real planet, but assumed to be moving at constant speed.
2. A situation in which a classical theory has a symmetry but the corresponding quantum theory does not. There are several types of quantum anomaly. Examples of the consequences of anomalies are that the number of types of lepton must equal the number of types of quark and that only certain groups are viable for superstring theory.