Source : Webster's Revised Unabridged Dictionary (1913)

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

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

            Any previous counsel or preventive understanding.
                                                  --Cudworth.

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

            Physic is either curative or preventive. --Sir T.
                                                  Browne.

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

Source : WordNet®

preventive
     adj 1: preventing or contributing to the prevention of disease;
            "preventive medicine"; "vaccines are prophylactic"; "a
            prophylactic drug" [syn: {preventative}, {prophylactic}]
     2: tending to prevent or hinder [syn: {preventative}] [ant: {permissive}]
     3: tending to ward off; "the swastika...a very ancient
        prophylactic symbol occurring among all peoples"- Victor
        Schultze [syn: {cautionary}, {prophylactic}]

preventive
     n 1: remedy that prevents or slows the course of an illness or
          disease; "the doctor recommended several preventatives"
          [syn: {preventative}, {prophylactic}]
     2: any obstruction that impedes or is burdensome [syn: {hindrance},
         {hitch}, {preventative}, {encumbrance}, {incumbrance}, {interference}]
     3: an agent or device intended to prevent conception [syn: {contraceptive},
         {preventative}, {contraceptive device}, {prophylactic
        device}, {birth control device}]