parts per million
- noun a measure of the concentration of a substance in a gas, liquid or solid.
- A unit of proportion, equal to a given amount of anything per million of the same thing or something else. Used, for instance, as a measure of concentration or to quantify error. For example, nine atoms of xenon per million of air. Its abbreviation is ppm.
- noun an expression of the proportions of active and inactive particles in a solution.