The Collaborative International Dictionary of English v.0.48:

Defervescence \De`fer*ves"cence\, Defervescency
\De`fer*ves"cency\, n. [L. defervescere to grow cool.]
   1. A subsiding from a state of ebullition; loss of heat;
      lukewarmness.
      [1913 Webster]

            A defervescency in holy actions.      --Jer. Taylor.
      [1913 Webster]

   2. (Med.) The subsidence of a febrile process; as, the stage
      of defervescence in pneumonia.
      [1913 Webster]

WordNet (r) 3.0 (2006):

defervescence
    n 1: abatement of a fever as indicated by a reduction in body
         temperature