What is 'magnitude'?

Magnitude may refer to: magnitude scale which helps people measure such earthquakes, tsunami and even volcanos

* Magnitude (mathematics), a measure of the size of a mathematical object:
o A vector object has both magnitude and direction as its defining characteristics.
o A scalar object has only magnitude as its defining characteristic.

In astronomy:

* A logarithmic measure of the brightness of an astronomical object:
o Ancient magnitude system
o Modern magnitude system
+ Apparent magnitude
# Apparent Photographic magnitude (Symbol mpg)
# Apparent Photovisual magnitude (Symbol mpv)
# Apparent Bolometric magnitude (Symbol mb)
+ Absolute magnitude
# Absolute Photographic magnitude (Symbol Mpg)
# Absolute Photovisual magnitude (Symbol Mpv)
# Absolute Bolometric magnitude (Symbol Mb)
* Magnitude of an eclipse

In seismology:

* Either of two measures of earthquake strength:
o Richter magnitude scale (including the surface-wave Ms and body wave Mb magnitudes)
o Moment magnitude scale

Magnitude is a measured value, as of size, brightness, or energy. 
Mathematics
Magnitude is the size (length) of a vector, or a general numerical size.

Seismic Events
Magnitude is the measure of the energy released by an earthquake; determined at the epicenter, and based on the height of the lines of a seismograph. This is a general idea of the intensity of an earthquake. 

Astronomy
Magnitude is the relative brightness (or rather dimness) of objects as seen from Earth. The higher the magnitude, the lower the comparative brightness of the star or planetary body. A separate use is to express the degree of solar obscuration in a solar eclipse.