The Collaborative International Dictionary of English v.0.48:

Phrenitis \Phre*ni"tis\, n. [L., fr. Gr. freni^tis, fr. frh`n,
   freno`s.]
   1. (Med.) Inflammation of the brain, or of the meninges of
      the brain, attended with acute fever and delirium; --
      called also cephalitis.
      [1913 Webster]

   2. See Frenzy.
      [1913 Webster]

WordNet (r) 3.0 (2006):

phrenitis
    n 1: inflammation of the brain usually caused by a virus;
         symptoms include headache and neck pain and drowsiness and
         nausea and fever (`phrenitis' is no longer in scientific
         use) [syn: encephalitis, cephalitis, phrenitis]