$$\require{cancel}$$
There is little to be said about density other than to define it as mass per unit volume. However, this expression does not literally mean the mass of a cubic metre, for after all a cubic metre is a large volume, and the density may well vary from point to point throughout the volume. Density is an intensive quantity in the thermodynamical sense, and is defined at every point. A more exact definition of density, for which I shall usually use the symbol $$\rho$$, is
$\rho=\lim_{\delta V\rightarrow 0}\frac{\delta m}{\delta V}. \label{16.2.1}$