Magnitude n. : Extent of dimensions; size; -- applied to things that have length, breath, and thickness.

Magnitude n. : That which has one or more of the three dimensions, length, breadth, and thickness.

Magnitude n. : Anything of which greater or less can be predicated, as time, weight, force, and the like.

Magnitude n. : Greatness; grandeur.

Magnitude n. : Greatness, in reference to influence or effect; importance; as, an affair of magnitude.

Example:
How much larger is a Magnitude 8 earthquake than a Magnitude 6?

Taxonomy
magnitude, apparent magnitude, Maximum brightness, brightness, Sun, full moon, variation, magnitude star, naked eye, Polaris, first magnitude, ratio, visible light, Sirius, free encyclopedia, measure, logarithmic scale, brightest star.

Defination from the web:
magnitude (m) of a celestial body is a measure of its brightness as seen by an observer on Earth, normalized to the value it would ... Each grade of magnitude was considered to be twice the brightness of the following grade (a logarithmic scale) ...