Boric acid ($H_3BO_3$ or $B(OH)_3$) is not a typical protonic acid (Arrhenius or Brønsted-Lowry acid) because it does not ionize to give up a proton on its own. Instead, it acts as a Lewis acid due to the electron-deficient boron atom. It accepts an electron pair from the $OH^-$ ion of a water molecule, completing its octet to form the tetrahydroxoborate ion $[B(OH)_4]^-$, and in the process, releases a proton ($H^+$) from the water molecule. The reaction is: $B(OH)_3 + 2H_2O \rightleftharpoons [B(OH)_4]^- + H_3O^+$.