Physicists and engineers use the phrase "order of magnitude" to refer to the smallest power of ten needed to represent a quantity. Two quantities and which are within about a factor of 10 of each other are then said to be "of the same order of magnitude," written .

Hardy and Wright (1979, p. 7) use the term to mean asymptotic to.

Hardy, G. H. and Wright, E. M. An Introduction to the Theory of Numbers, 5th ed. Oxford, England: Clarendon Press, 1979.

Jeffreys, H. and Jeffreys, B. S. "Orders of Magnitude." §1.08 in Methods of Mathematical Physics, 3rd ed. Cambridge, England: Cambridge University Press, pp. 23-24, 1988.

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Weisstein, Eric W. "Order of Magnitude." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/OrderofMagnitude.html