An Etymological Dictionary of Astronomy and Astrophysics

A fraction of a whole number in units of 1/1000,000.
It is usually used to describe chemical concentrations, very small amounts of pollutants in air, water, body fluids, and uncertainty. For example 30 ppm is 3 x 10^-5 or 0.003%.

See also: → part; → per; → million.