Pure substance is a chemical description of a substance or matter containing only one element, literally a 'pure substance'. The importance of this definition is that it refers to a specific set of characteristics and known chemical and physical properties. A variation on this definition is sometimes used in cases where there's more than one element present, but the requirement for known properties is met by the compound.
Examples of Pure Substance:
Copper carbonate: referred to as a pure substance because of the common composition of all forms of the material.
Gold: It's a naturally non-allotropic material, homogeneous in composition.