Parts per million (parts in 106 parts) e.g. the number of parts of active ingredient per million parts of the substance in question. They may include residues in soil, water or whole animals. 10 ppm is equivalent to 0.001%.
Swimming pool chemistry: Parts per million. A method of assigning value to certain concentrations of chemicals in the water. For example, alkalinity should be kept at 80-120 parts per million, by weight and in relation to the water it's dissolved in.
Found on http://www.1st-direct.com/acatalog/Chemical_Glossary.html