What is Soft Water?

Legal Definition
Soft water is surface water that contains low concentrations of ions and in particular is low in ions of calcium and magnesium. Soft water naturally occurs where rainfall and the drainage basin of rivers are formed of hard, impervious and calcium poor rocks. Examples in the UK include Snowdonia in Wales and the Western Highlands in Scotland.

The term may also be used to describe water that has been produced by a water softening process although such water is more correctly termed softened water. In these cases the water may also contain elevated levels of sodium and/or bicarbonate ions .

Because soft water has few calcium ions, there is no inhibition of the lathering action of soaps and no soap scum is formed in normal washing. Similarly, soft water produces no calcium deposits in water heating systems. Water that is not soft is referred to as hard water.

In the UK, water is regarded as soft if the hardness is less than 50 mg/l of calcium carbonate. Water containing more than 50 mg/l of calcium carbonate is termed hard water. In the USA soft water is classified as having less than 60 mg/l of calcium carbonate.
-- Wikipedia
Legal Definition
Water that does not containe high levels of dissolved solids such as minerals that make it hard to lather. Soft water lathers easily.