The purest water that is theoretically possible would consist of nothing but H2O molecules and exclude substances like minerals and salts. This maximum is never truly achieved for large samples in practice, so the term "pure water" has taken on different meanings that vary by the water's intended use.
The U.S. Environmental Protection agency sets standards for how many impurities can be present in potable water. These impurities are generally expressed in parts per million or parts per billion. Since 2006, arsenic, for example, can be present in water at a concentration of up to 10 parts per billion in otherwise "pure" drinking water.