1) A substance that produces H+(aq) ions in aqueous solution. Strong acids ionize completely or almost completely in dilute aqueous solution. Weak acids ionize only slightly.
2) Compounds that contain hydrogen (H #1) and when dissolved in water (H2O), they increase the concentration of hydrogen ions, H+ (hydronium ions H3O+). Acids are proton donors. Substances with a pH less than 7 are considered to be acidic.
3) A compound that yields H+ions in solution or a solution with the concentration of H+exceeding OH-.