Definition
One ppm is comparable to one drop of gasoline in a tankful of gas (full-size car).
See also:
Milligrams/liter (mg/l)
A measure of concentration used in the measurement of fluids. Mg/l is the most common way to present a concentration in water and is roughly equivalent to parts per million.
Parts per Billion (ppb)
One ppb is comparable to one kernel of corn in a filled, 45-foot silo, 16 feet in diameter.
Parts per Trillion (ppt)
One ppt is comparable to one drop in a swimming pool covering the area of a football field 43 ft. deep.