1. a degree in a continuum of size or quantity

e.g. it was on the order of a mile
an explosion of a low order of magnitude

Synonym: order

2. 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

Synonym: magnitude