The Collaborative International Dictionary of English v.0.48:

Periodic \Per`i*od"ic\ (p[~e]r`[-i]*[o^]d"[i^]k), a. [Pref. per-
   + iodic.] (Chem.)
   Pertaining to, derived from, or designating, the highest
   oxygen acid (HIO4) of iodine.
   [1913 Webster] Periodic