Ionization is a special case of dissociation.
Dissociation is a process in which compounds separate or split into smaller particles.
We might write the general equation as
In this case, the solid
All they are doing when
When the substance already contains ions, the process is often called dissociation.
Many covalent compounds also dissociate in water to form ions.
When covalent compounds dissolve in water to produce ions, the dissociation process is often called ionization.
Here's a great video on ionization and dissociation.