preventive

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Intended or used to prevent or hinder; acting as an obstacle.
• adjective Carried out to deter expected aggression by hostile forces.
• adjective Preventing or slowing the course of an illness or disease; prophylactic.
• noun Something that prevents; an obstacle.
• noun Something that prevents or slows the course of an illness or disease.

from The Century Dictionary.

• Serving to prevent or hinder; guarding against or warding off something, as disease, injustice, loss, etc.
• noun That which goes before; an anticipation.
• noun That which prevents; that which constitutes an effectual check or insurmountable obstacle.
• noun Specifically, something taken, used, or done beforehand to ward off disease.
• noun Also preventative.

from the GNU version of the Collaborative International Dictionary of English.

• noun That which prevents, hinders, or obstructs; that which intercepts access; in medicine, something to prevent disease; a prophylactic.
• adjective obsolete Going before; preceding.
• adjective Tending to defeat or hinder; obviating; preventing the access of.
• adjective [Eng] the duty performed by the armed police in guarding the coast against smuggling.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Preventing, hindering, or acting as an obstacle to.
• adjective Carried out to deter military aggression.
• adjective Slowing the development of an illness; prophylactic.
• noun nonstandard A thing that prevents, hinders, or acts as an obstacle to.
• noun nonstandard A thing that slows the development of an illness.
• noun A contraceptive, especially a condom.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun remedy that prevents or slows the course of an illness or disease
• adjective tending to prevent or hinder
• adjective preventing or contributing to the prevention of disease
• noun any obstruction that impedes or is burdensome
• noun an agent or device intended to prevent conception