The Collaborative International Dictionary of English v.0.48:

Vicissitude \Vi*cis"si*tude\, n. [L. vicissitudo, fr. vicis
   change, turn: cf. F. vicissitude. See Vicarious.]
   [1913 Webster]
   1. Regular change or succession from one thing to another;
      alternation; mutual succession; interchange.
      [1913 Webster]

            God made two great lights . . .
            To illuminate the earth and rule the day
            In their vicissitude, and rule the night. --Milton.
      [1913 Webster]

   2. Irregular change; revolution; mutation.
      [1913 Webster]

   3. (pl.) Changing conditions of fortune in one's life; life's
      ups and downs.
      [PJC]

            This man had, after many vicissitudes of fortune,
            sunk at last into abject and hopeless poverty.
                                                  --Macaulay.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vicissitude
    n 1: a variation in circumstances or fortune at different times
         in your life or in the development of something; "the
         project was subject to the usual vicissitudes of
         exploratory research"
    2: mutability in life or nature (especially successive
       alternation from one condition to another)