The Collaborative International Dictionary of English v.0.48:

Lysis \Ly"sis\ (l[imac]"s[i^]s), n. [NL., fr. Gr. ly`sis.]
   (Med.)
   The resolution or favorable termination of a disease, coming
   on gradually and not marked by abrupt change.
   [1913 Webster]

   Note: It is usually contrasted with crisis, in which the
         improvement is sudden and marked; as, pneumonia ends by
         crisis, typhoid fever by lysis.
         [1913 Webster]

WordNet (r) 3.0 (2006):

lysis
    n 1: recuperation in which the symptoms of an acute disease
         gradually subside
    2: (biochemistry) dissolution or destruction of cells such as
       blood cells or bacteria