mag·ni·tude / ˈmagnəˌtoōd/ • n. 1. the great size or extent of something: they may feel discouraged at the magnitude 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 astronomical object 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 visible to 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.PHRASES: of the first magnitudesee first.