Ppm
Ppm or parts per million is a dimensionless quantity used to describe the ratio of an amount of substance within another substance, specifically denoting 1 part per 1 000 000 parts. It is used similarly to percents (%) which are in fact just a measure of a part per hundred (1% = pph), however it compares quantities on a much smaller scale. For instance, 32 seconds would be roughly equivalent to one ppm of a year,[1] and a millimeter is one ppm of a kilometer.
Ppm is commonly used to describe quantities of trace substances in order to communicate abundance; such as carbon dioxide in the atmosphere or the abundance of uranium within the Earth's crust.