Parts per Million:

Another word used to define the specific concentration of a solution is category per million or ppm. The word ppm is described as the concentration of a solution within units of one category of solute to one million parts solvent. A single ppm equals one milligram of solute per liter of solution. Other word, parts per billion (ppb), is described as one part solute per one billion parts solvent. A single ppb is equivalent to one microgram solute per liter of solution. Those two words are commonly used for extremely dilute solutions.