The Collaborative International Dictionary of English v.0.48:

Preventive \Pre*vent"ive\, a. [Cf. F. pr['e]ventif.]
   1. Going before; preceding. [Obs.]
      [1913 Webster]

            Any previous counsel or preventive understanding.
                                                  --Cudworth.
      [1913 Webster]

   2. Tending to defeat or hinder; obviating; preventing the
      access of; as, a medicine preventive of disease.
      [1913 Webster]

            Physic is either curative or preventive. --Sir T.
                                                  Browne.
      [1913 Webster]

   Preventive service, the duty performed by the armed police
      in guarding the coast against smuggling. [Eng]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Preventive \Pre*vent"ive\, n.
   That which prevents, hinders, or obstructs; that which
   intercepts access; in medicine, something to prevent disease;
   a prophylactic.
   [1913 Webster]

WordNet (r) 3.0 (2006):

preventive
    adj 1: preventing or contributing to the prevention of disease;
           "preventive medicine"; "vaccines are prophylactic"; "a
           prophylactic drug" [syn: preventive, preventative,
           prophylactic]
    2: tending to prevent or hinder [syn: preventive,
       preventative] [ant: permissive]
    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]