The Collaborative International Dictionary of English v.0.48:

Polybasic \Pol`y*ba"sic\, a. [Poly- + basic.] (Chem.)
   Capable of neutralizing, or of combining with, several
   molecules of a monacid base; having several hydrogen atoms
   capable of being replaced by basic radicals; -- said of
   certain acids; as, sulphuric acid is polybasic.
   [1913 Webster]