Symantics of parts per (million, billion, etc)
The notation "parts per" is just a dimensionless unit to describe small amounts.
In chemistry it's used for concentrations of dilute liquids.. whereas in physics it can be used for defining change with respect to another quantity and in Six sigma it's used to define "defects per million opportunities".

In chemistry however, $ppm$ is mostly used as concentrations of dilute solutions in water and sometimes used interchangeable with $mg/l$. The notion of "one particle per million particles of solvent" can be used for solid-solid solutions or initial understanding of the term.