ETYM Latin magnitudo, from magnus great. Related to Master, Maxim.
The size of a number, regardless of its sign (+ or –). For example, 16 and –16 have the same magnitude. See also absolute value.
1. Relative importance.
2. The property of relative size or extent.

Size irrespective of sign, used especially for vectors irrespective of direction.

In astronomy, measure of the brightness of a star or other celestial object. The larger the number denoting the magnitude, the fainter the object. Zero or first magnitude indicates some of the brightest stars. Still brighter are those of negative magnitude, such as Sirius, whose magnitude is -1.46. Apparent magnitude is the brightness of an object as seen from Earth; absolute magnitude is the brightness at a standard distance of 10 parsecs (32.6 light-years).
Each magnitude step is equal to a brightness difference of 2.512 times. Thus a star of magnitude 1 is (2.512)5 or 100 times brighter than a sixth-magnitude star just visible to the naked eye. The apparent magnitude of the Sun is -26.8, its absolute magnitude +4.8.