What is "sodium acid carbonate"

Sodium bicarbonate \Sodium bicarbonate\, a white crystalline substance, HNaCO3, with a slight alkaline taste resembling that of sodium carbonate. It is found in many mineral springs and also produced artificially,. It is used in cookery, in baking powders, and as a source of carbonic acid gas (carbon dioxide) for soda water. Called also baking soda, cooking soda, bicarbonate of soda, bicarb, saleratus, and technically, acid sodium carbonate, sodium acid carbonate, primary sodium carbonate, sodium dicarbonate, etc.