order of magnitude

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An estimate of size or magnitude expressed as a power of ten.
• noun A range of values between a designated lower value and an upper value ten times as large.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The class of scale or magnitude of any amount, where each class contains values of a fixed ratio (most often 10) to the class preceding it. For example, something that is 2 orders of magnitude larger is 100 times larger, something that is 3 orders of magnitude larger is 1000 times larger, and something that is 6 orders of magnitude larger is a million times larger, because = 100, = 1000, and = a million.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• 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
• noun a degree in a continuum of size or quantity