Source : Webster's Revised Unabridged Dictionary (1913)

Hyperpyrexia \Hy`per*py*rex"i*a\, n. [NL. See {Hyper-}, and
   {Pyrexia}.] (Med.)
   A condition of excessive fever; an elevation of temperature
   in a disease, in excess of the limit usually observed in that
   disease.

Source : WordNet®

hyperpyrexia
     n : extremely high fever (especially in children)