- having more than one replaceable hydrogen atom.
- capable of reacting with more than one equivalent weight of an acid.
- an acid having more than one replaceable hydrogen atom.
- an oxyacid containing the equivalent of extra molecules of its anhydride, as the polysulfuric acids, H2S2O7 or H2SO4⋅SO3, and H2S3O10 or H2SO4⋅2SO3.
