What is a Substance ?
Chemistry is the study of physical matter, which may be classified in many different ways, such as state of matter (gas, liquid or solid), chemical form (element, mixture or compound), chemical structure (atoms or molecules, etc.) and so on.
In addition to 'element', 'mixture' and 'compound', another useful term is the word 'substance', which can be used to refer to either an element or a compound - but not to a mixture because a 'substance' always has a definite composition.
The relationship between these terms may be summarized in the form of a diagram:
Definitions of a Chemical Substance:
Chemical substances are any materials (in any state - solid, liquid or gas) that have a definite chemical composition.
Chemical substances can therefore be either a pure chemical element or a pure chemical compound.
This definition and interpretation is generally considered sufficient for school-level, incl GCSE Chemistry.
There are, however, some other definitions e.g. "A form of matter that has both definite composition and distinct properties. A pure substance cannot be separated into simpler components without chemical change. Physical changes can alter the state of matter but not the chemical identity of a pure substance.", diracdelta science & engineering encyclopedia (www.diracdelta.co.uk/science/source/p/u/pure%20substance/source.html).
Examples of Substances:
Element / Mixture* / Compound
Exists as atoms / molecules
Usual state under standard conditions
See also the page about Elements Mixtures and Compounds, and Atoms and Molecules.