(1) The physical matter, material or that of which an organ or body consists.
(2) (chemistry) A material, matter or that which has definite chemical composition and distinct properties.
(3) The most vital or essential part of something.
(4) Drugs, as in substance abuse.
In chemistry, all compounds are substances but not all substances are compounds since pure elements are also chemical substances. Chemical compounds may exist as solid, liquid or gas. Examples are water, salt, sugar, etc. Forms of energy such as light and heat are not considered a chemical substance.