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]

Moby Thesaurus II by Grady Ward, 1.0:

146 Moby Thesaurus words for "preventive":
   aegis, antacid, anticipative, anticipatory, antidote, arm guard,
   backstop, barrier, block, buffer, bulwark, bumper, contraceptive,
   copyright, counteractant, counteractive, counteragent,
   counterirritant, countermeasure, covering, crash helmet, curb,
   cushion, custodial, dashboard, defensive, deterrent, deterring,
   discouraging, dodger, ethnocentric, exceptional, excluding,
   exclusive, exclusory, face mask, fender, finger guard, foot guard,
   forbidding, forestalling, fostering, fuse, goggles, governor,
   guard, guardian, guarding, guardrail, hampering, hand guard,
   handrail, hard hat, helmet, hindering, hindrance, immunizing,
   impediment, impeding, inadmissible, inhibiting, inhibition,
   inhibitive, inhibitory, inoculant, inoculum, insular, insulation,
   interdictive, interdictory, interlock, knee guard, knuckle guard,
   laminated glass, life preserver, lifeline, lightning conductor,
   lightning rod, mask, mudguard, narrow, neutralizer, nose guard,
   nullifier, obstacle, obstruction, offset, pad, padding, palladium,
   parental, parochial, patent, pilot, precautionary, preclusive,
   prescriptive, preventative, prevention, prohibiting, prohibitive,
   prohibitory, prophylactic, proscriptive, protecting, protection,
   protective, protective clothing, protective umbrella, remedy,
   repressive, restraining, restrictive, safeguard, safeguarding,
   safety, safety glass, safety plug, safety rail, safety shoes,
   safety switch, safety valve, screen, screening, seat belt,
   seclusive, segregative, select, selective, separative, serum,
   sheltering, shield, shielding, shin guard, snobbish, sun helmet,
   suppressive, tutelary, umbrella, vaccine, vigilant, watchful,
   windscreen, windshield, xenophobic