Simple Definition:

An ion is an atom or a molecule that has an electric charge. Atoms usually have an equal number of electrons [tiny particles with negative charge] and protons [tiny particles with positive charge], so they have no charge overall. But sometimes an atom can gain or lose an electron, which makes it have a charge. When an atom gains an electron, it becomes a negatively charged ion called an anion. When an atom loses an electron, it becomes a positively charged ion called a cation. Ions are important in chemistry because they can react with other ions to form new substances.

Very Simple Definition:

An ion is a tiny thing that can have an electric charge. Sometimes, atoms can gain or lose an electron, which makes them have a charge. When an atom has a charge, it is called an ion.