The Collaborative International Dictionary of English v.0.48:

Prophylaxis \Proph`y*lax"is\, n. [NL. See Prophylactic.]
   (Med.)
   The art of preserving from, or of preventing, disease; the
   observance of the rules necessary for the preservation of
   health; preservative or preventive treatment.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prophylaxis
    n 1: the prevention of disease