A substance is simply matter with definite chemical composition and distinct properties. It is matter that is characterized by a constant composition in terms of its molecules, formulae and atoms, as well as physical properties such as density, refractive index, electric conductivity, melting point, and so on.
A substance can be an element or a compound but NOT a mixture. It can also be matter that exists in its pure form, usually called a pure substance. A few examples of substances include Water (), Hydrogen () and Neon (Ne).
Other examples of chemical substances commonly seen in pure form are salt (sodium chloride), diamond (carbon) and gold.