In mathematics, magnitude is the size of a mathematical object, a property which determines whether the object is larger or smaller than other objects of the same kind. More formally, an object's magnitude is the displayed result of an ordering of the class of objects to which it belongs.

Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking caused by an earthquake at a given location.

Apparent magnitude is a measure of the relative brightness of a star or other astronomical objects as seen by an observer. An object's apparent magnitude depends on its intrinsic luminosity, its distance, and any extinction of the object's light by interstellar dust along the line of sight to the observer.

The moment magnitude scale is a measure of an earthquake's magnitude based on its seismic moment, expressed in terms of the familiar magnitudes of the original "Richter" magnitude scale.