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) say a real function and positive function with continuous variable that tends to some limit are of the same order of magnitude, written using asymptotic notation as , if for positive constants and independent of . This term is also used for an integer variable that tends to infinity, a real function , and a positive function .