This is is the common name for the aqueous cation h3o+, formed when a water molecule combines with the proton from a hydrogen atom.

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).