magnitude |ˈmagnəˌtoōd|noun1 the great size or extent of something : they may feel discouraged at themagnitude of the task before them.• great importance : events of tragic magnitude.2 size : electorates of less than average magnitude.• a numerical quantity or value : the magnitudes of all the economic variables could be determined.3 the degree of brightness of a star. The magnitude of an astronomicalobject is now reckoned as the negative logarithm of the brightness; a decrease of one magnitude represents an increase in brightness of 2.512 times. A star with an apparent magnitude of six is barely visibleto the naked eye. See also apparent magnitude, absolute magnitude .• the class into which a star falls by virtue of its brightness.• a difference of one on a scale of brightness, treated as a unit of measurement.PHRASESof the first magnitude see first .ORIGIN late Middle English (also in the sense [greatness of character] ): from Latin magnitudo, from magnus ‘great.’