This is an abbreviation for "parts per million" and it also can be expressed as milligrams per liter (mg/L). This measurement is the mass of a chemical or contaminate per unit volume of water. Seeing ppm or mg/L on a lab report means the same thing.
One ppm is equivalent to the absolute fractional amount multiplied by one million. A better way to think of ppm is to visualize putting four drops of ink in a 55-gallon barrel of water and mixing it thoroughly. This procedure would produce an ink concentration of 1 ppm. Some other analogies that may help you visualize the scale involved with ppm. One ppm is like:
An even smaller concentration measurement is parts per billion (ppb). One ppb is one part in 1 billion. Because a ppb is a much lower concentration, some analogies would be :