The total amount of organic and inorganic substances dissolved in water. The principal constituents in fresh water are sodium, potassium, calcium, and magnesium, sodium, and potassium cations, and carbonate and hydrogen carbonate, chloride, sulphate, and nitrate anions. It is a measure of the quality of water for drinking and for rivers. The main source of total dissolved solids is from sewers, urban sources, and agricultural surface runoff. The total dissolved solids are measured in parts per million (ppm). The World Health Organization considers drinking water to be unacceptable if the total dissolved solids content is in excess of 1,200 ppm. Compare total suspended solids.

http://www.who.int/water_sanitation_health/dwq/chemicals/tds.pdf Official website of the World Health Organization information on water sanitation.