Well, according to two fairly authoritative sources on the web substance
would generally be equivalent to pure substance, but with the following note
in the Wikipedia:
Chemical substances (also sometimes referred to as a pure substances) are
often defined as "any material with a definite chemical composition" in most
introductory general chemistry textbooks.[3] According to this definition a
chemical substance can either be a pure chemical element or a pure chemical
compound. However, there are exceptions to this definition; a pure substance
can also be defined as a form of matter that has both definite composition
and distinct properties.[4] The chemical substance index published by CAS
also includes several alloys of uncertain composition.[5] Non-stoichiometric
compounds are a special case (in inorganic chemistry) that violates the law
of constant composition, and for them, it is sometimes difficult to draw the
line between a mixture and a compound, as in the case of palladium hydride.