The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hyperpyrexia
    n 1: extremely high fever (especially in children)