The definition that most consistently applies is that a varnish is a protective coating comprised of one or more resins dissolved into a solution which can easily be applied to a surface.
Reactive coatings cure by chemical reaction, oxygenation, moisture, radiation, etc.
Evaporative coatings, like shellac, can be reversed at any time by simply using one of the main solvents originally used to keep the coating in a workable, liquid state. Reactive coatings, which include oil-based varnishes, must be removed with strong solvents after they are cured, i.e. with paint remover. The original varnishes were natural resins dissolved into spirits of wine, i.e. crude ethanol. Shellac, mastic, sandarac, and manila copal are all examples of these alcohol soluble types, but so are rosin and dammar which are soluble in turps. Folks were puzzled as to why some very similar looking, hard resins did not dissolve in alcohol or turps.