The characteristic of a substance that is dependent on the electronic configuration of its valence electrons.
This is to distinguish “chemical properties” from “physical ones” that may also be related to the chemical composition. If a change does not involve rearrangement of the valence electrons, it is not chemical.
For example, mass, density, and hardness are all physical properties, but acidity, magnetism, flammability and other reactivity are all chemical properties. Crushing a rock does not make the parts less dense or hard. Dissolving a mineral rock changes the chemical structure by breaking existing atomic linkages (electron bonding) and reforming/recombining them into different electron bonds or ions.
Similarly, a physical change is breaking a piece of wood. A chemical change is burning it. When something is burned we see a “physical” change, but the real change is the changing of the carbon-carbon-hydrogen bonds (in wood, for example) to carbon-oxygen and hydrogen-oxygen bonds.