1. n. 
Abbreviation for the expression of concentration, parts per million. For solid and liquid concentrations, ppm refers to weight (mass) units. For example:
The relationship of ppm to weight percentage is 10,000 ppm = 1 wt%. For gases, ppm refers to volume (or mole) units. For example, H2S in an air sample may be 10 ppm (both by volume and by moles).