Total Dissolved Solids is a measure of the combined content of all inorganic and organic

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

 substances contained in a liquid in: molecular, ionized or micro-granular (colloidal sol

Sol (colloid)

A sol is a colloidal suspension of very small solid particles in a continuous liquid medium. They are quite stable and show the Tyndall effect. Examples include blood, pigmented ink, and paint....

) suspended form. Generally the operational definition is that the solids must be small enough to survive filtration through a sieve the size of two micrometer. Total dissolved solids are normally discussed only for freshwater systems, as salinity comprises some of the ions constituting the definition of TDS.