Parts-per notation

In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Commonly used are ppm (parts-per-million, 10–6), ppb (parts-per-billion, 10–9), ppt (parts-per-trillion, 10–12) and ppq (parts-per-quadrillion, 10-15).