An electrolyte is a liquid that contains ions and conducts electricity. The substance inside a battery is an electrolyte.
In chemistry, an electrolyte is typically dissolved in a solvent, becoming ionized — gaining a positive or negative charge. The resulting solution can conduct electricity when voltage is applied to it, making electrolytes especially useful in batteries. The word electrolyte was coined in the 1800s from electro-, "electrical," from the Greek root elektro, and lytos, or "loosed" in Greek.