The term magnetic anomaly is used for bodies (The Earth, other planets, moons) to indicate a region/volume where the magnetic field does not fit the overall pattern. They generally indicate the presence of some material that holds magnetism (e.g., iron), and some past cause, a time when a magnetic field other than the current overall field was present. Alternately, one could indicate an active dynamo other than an "overall" dynamo centered on the body.
In the case of a body with nearly no magnetic field, such as the Moon, the mere existence of magnetism in some locality constitutes an anomaly. Otherwise, e.g., on Earth, it is magnetic field lines pointing in some direction other than that of the body's overall field.