Definition of "Magnitude" :

noun: the property of relative size or extent (whether large or small)

"They tried to predict the magnitude of the explosion."

noun: relative importance

"A problem of the first magnitude."

noun: a number assigned to the ratio of two quantities; two quantities are of the same order of magnitude if one is less than 10 times as large as the other; the number of magnitudes that the quantities differ is specified to within a power of 10