 The term ‘nominal’ refers to names or naming, and is used by Locke to indicate that classifying substances based on their similar qualities is an exercise in naming. For example, the nominal essence of gold is the abstract idea that constitutes our definition of ‘gold’, i.e., yellowish, heavy, malleable metal that can be dissolved in Aqua Regia, but not in sulfuric acid, etc. According to Locke, the nominal essence contains the properties that give meaning to the name ‘gold’ and allow us to know what gold is wherever we find it. What makes the nominal essence an essence is that it contains both the necessary and sufficient conditions for something to belong to its species or genus. Because of how gold is defined, every yellowish, heavy, malleable metal that dissolves in water and hydrochloric acid and not in sulfuric acid, is gold.