Source : Webster's Revised Unabridged Dictionary (1913)

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.

            A defervescency in holy actions.      --Jer. Taylor.

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

Source : WordNet®

defervescence
     n : abatement of a fever