An Etymological Dictionary of Astronomy and Astrophysics

In general, a deviation from the norm.
An angle that gives the position of an object in an elliptical orbit at any given time with respect to its primary. The true anomaly is the angle between the periapsis of an orbit and the object’s current orbital position, measured from the body being orbited and in the direction of orbital motion. The mean anomaly is what the true anomaly would be if the object orbited in a perfect circle at constant speed.

Etymology (EN): Anomaly from → anomalous

Etymology (PE): Nâsâni, from nâ- negation suffix + sân “rule, law, custom” + -i noun maker suffix.