Answer is: hydronium ion.
Chemical reaction: H⁺(aq) + H₂O(l) → H₃O⁺(aq).
Hydronium ion is positive ion present when an Arrhenius acid, for example hydrochloric acid (HCl(aq)) is dissolved in water, hydrochloric acid in solution give up a proton (a positive hydrogen ion, H⁺) to the surrounding water molecules (H₂O): HCl(aq) + H⁺(aq) → Cl⁻(aq) + H₃O⁺(aq).
