The Collaborative International Dictionary of English v.0.48:

Convalesce \Con`va*lesce"\, v. i. [imp. & p. p. Convalesced;
   p. pr. & vb. n. Convalescing.] [L. convalscere; con- +
   valescere to grow strong, v. incho. of valere to be strong.
   See Valiant.]
   To recover health and strength gradually, after sickness or
   weakness; as, a patient begins to convalesce.
   [1913 Webster]