Stands for parts per million, a chemical term referring to quantity of particular chemical either in solution or in a gas.
A measure of proportion by weight which is equivalent to one unit weight of solute (dissolved substance) per million unit weights of the solution. Since one litre of water weighs one million milligrams, one ppm is equal to one milligram per litre (mg/L). Ppm is the preferred unit of measure in water or wastewater analysis.