A medium (phase) which conducts electricity by the movement of ions.
Molten sodium chloride but not molten wax.
A substance which dissolves to give a conducting solution.
sodium chloride dissolves in water to give an aqueous solution containing Na+ and Cl¯ ions.
An aqueous solution of any soluble ionic compound is an electrolyte.
Hydrochloric acid, an aqueous solution of hydrogen chloride is a strong electrolyte; liquid hydrogen chloride is not an electrolyte.
A sugar solution is not an electrolyte, sugar does not form ions in water.