What distinguishes ionization, dissociation, and dissolution?

Here are the definitions I use.

Ionization is the conversion of a neutral molecule or atom to an ion.

Thus, the conversion of an ##"Na"## atom to ##"Na"^+## is an ionization.

When ##"HCl"## gas dissolves in water, it reacts to form separate ##"H"_3"O"^+## and ##"Cl"^"-"## ions.

##"HCl(g)" + "H"_2"O(l)" → "H"_3"O"^+"(aq)" + "Cl"^"-""(aq)"##

This is also a dissociation process.

Dissociation is a process in which a compound separates into two or more parts.

For example, water dissociates into hydronium and hydroxide ions.

When ##"NaCl"## dissolves in water, it forms separate ##"Na"^+## and ##"Cl"^"-"## ions. This is a dissociation process.

##"NaCl(s)" → "Na"^"+""(aq)" + "Cl"^"-""(aq)"##

Here's a great video that shows how water dissolves salt.

##"N"_2"O"_4## decomposes into two ##"NO"_2## molecules. This is a dissociation process.

##"O"_2"N-NO"_2 → "2NO"_2##

Dissolution is the process in which the particles of a substance move into the solvent.

The process may not involve dissociation, as in the dissolving of sugar …

(from antoine.frostburg.edu)

… or it may, as in the case of sodium chloride.

Sugar does not dissociate in water, but ##"NaCl"## does dissociate into ions when it dissolves.