TDS (total dissolved solids)

The total amount of minerals, salts and other substances dissolved in water, measured in milligrams per litre. Handheld meters estimate TDS from electrical conductivity; a laboratory measures it by evaporating a filtered sample and weighing the residue.

High TDS affects taste and can point to salty or mineral-rich groundwater. Very low TDS, common after RO treatment, is not a sign of safety on its own, since TDS says nothing about bacteria.