Great importance or consequence: affairs of magnitude.

Greatness of size or amount.

Moral greatness: magnitude of mind.

Astronomy.

In mathematics:

Magnitude (mathematics), the relative size of a mathematical object.
Magnitude (vector), a term for the size or length of a vector.
Scalar (mathematics), a quantity defined only by its magnitude.
Euclidean vector, a quantity defined by both its magnitude and its direction.
Order of magnitude, the class of scale having a fixed value ratio to the preceding class.

In astronomy:

Magnitude (astronomy), a measure of brightness and brightness differences used in astronomy.
Apparent magnitude, the apparent brightness of a celestial object as observed from Earth.
Absolute magnitude, the brightness of a celestial object corrected to a standard luminosity distance.
Photographic magnitude, the brightness of a celestial object corrected for photographic sensitivity, symbol mpg
Magnitude of eclipse or geometric magnitude, the size of the eclipsed part of the Sun during a solar eclipse or the Moon during a lunar eclipse.

As an earthquake unit of measure:

Richter magnitude scale,a measure of the energy of an earthquake.
Moment magnitude scale, an alternative to the Richter scale for measuring earthquakes.