The Collaborative International Dictionary of English v.0.48:

Preventative \Pre*vent"a*tive\, n.
   That which prevents; -- incorrectly used instead of
   preventive.
   [1913 Webster]

WordNet (r) 3.0 (2006):

preventative
    adj 1: tending to prevent or hinder [syn: preventive,
           preventative] [ant: permissive]
    2: preventing or contributing to the prevention of disease;
       "preventive medicine"; "vaccines are prophylactic"; "a
       prophylactic drug" [syn: preventive, preventative,
       prophylactic]
    n 1: remedy that prevents or slows the course of an illness or
         disease; "the doctor recommended several preventatives"
         [syn: preventive, preventative, prophylactic]
    2: any obstruction that impedes or is burdensome [syn:
       hindrance, hinderance, hitch, preventive,
       preventative, encumbrance, incumbrance, interference]
    3: an agent or device intended to prevent conception [syn:
       contraceptive, preventive, preventative, contraceptive
       device, prophylactic device, birth control device]