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

Origin of polyacid

First recorded in 1855–60; poly- + acid
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018