Parts Per Million
Parts per Million (aka PPM or ppm)
Parts-per notation is used, especially in science and engineering, to denote relative proportions in measured quantities; particularly in low-value (high-ratio) proportions at the parts-per-million (ppm) 10–6, parts-per-billion (ppb) 10–9, and parts-per-trillion (ppt) 10–12 level. Since parts-per notations are quantity-per-quantity measures, they are known as dimensionless quantities; that is, they are pure numbers with no associated units of measurement. In regular prose, parts-per notations generally take the literal “parts per” meaning of a comparative ratio. However, in mathematical expressions, parts-per notations function as coefficients with values less than 1.
While many disciplines see this as 'their' measurement (such as automotive), it is used in all science disciplines.