Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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