A mathematical term used loosely to indicate tenfold differences between values. This concept is crucial to interpreting logarithmic scales such as pH or earthquake magnitude, where each number differs by a factor of 10, and two numbers differ by a factor of 100 (10 times 10). For example, a pH of 4 is 100 times as acidic as a pH of 6 because they differ by two orders of magnitude. Scientists often generalize with this term; for example "our ability to measure pollutants has improved by several orders of magnitude" means that whereas before we were able to measure parts per million (ppm), we can now measure parts per billion (ppb).