Material Some objects are made of material that seems to have a consistent compositon throughout. This might be a metal, glass, marble, plastic, paper, etc. The composition varies throughout a heterogenous material.
Clear solutions are homogenous mixtures as is the air.
As such a substance is homogeneous. A salt solution is homogeneous but is not a substance because the chemical composition is quite arbitrary. It can range from very dilute to concentrated and still be a homogeneous material. A substance usually refers to materials such as salt, sugar, water, calcite, quartz, potassium chloride, all of which have definte ratios of their component elements. Nitrogen is a substance, air is not.