Let's look at NaCl as an example. When placed in water the solid disappears and the solution tastes salty and conducts electricity very well. Something obviously changed. Sodium and Chloride ions when dissolved in water. When solid ionic compounds separate into ions in a water solution, they are said to Dissociate .
Many ionic solids dissociate in water.
Also, compounds that aren't primarily ionic in nature form ions in solution. When molecular compounds separate into ions in solution, it is called Ionization . Notice the difference in electronegativity is only 0.9 so the bond is classed as mostly covalent. Sometimes all of these processes that lead to ion formation in solution are called dissociation. Technically, only those solids that were already ionically bonded are said to dissociate.