noun

• (chemistry) A substance that, in solution or when molten, ionizes and conducts electricity.
• (chemistry) A solution containing such substances, which therefore also conducts electricity.
• (chemistry, often) Any of the various ions in such a solution, or (metonymically) their concentration therein.
• (physiology, medicine) Any of the various biometal ions (such as sodium or chloride) that regulate the electric charge on cells and the flow of water across their membranes, or (metonymically) such an ion's concentration in a bodily fluid, usually and especially blood, blood serum, or urine.