The Collaborative International Dictionary of English v.0.48:

Prophylactic \Proph`y*lac"tic\, n. [Cf. F. prophylactique.]
   (Med.)
   A medicine which preserves or defends against disease; a
   preventive.
   [1913 Webster] Prophylactic

The Collaborative International Dictionary of English v.0.48:

Prophylactic \Proph`y*lac"tic\, Prophylactical
\Proph`y*lac"tic*al\, a. [Gr. ?, fr. ? to guard against; ?
   before + ? to guard: cf. F. prophylactique.] (Med.)
   Defending or preserving from disease; preventive. --Coxe.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prophylactic
    adj 1: capable of preventing conception or impregnation;
           "contraceptive devices and medications" [syn:
           contraceptive, prophylactic, antifertility]
    2: warding off; "the swastika...a very ancient prophylactic
       symbol occurring among all peoples"- Victor Schultze [syn:
       cautionary, prophylactic]
    3: preventing or contributing to the prevention of disease;
       "preventive medicine"; "vaccines are prophylactic"; "a
       prophylactic drug" [syn: preventive, preventative,
       prophylactic]
    n 1: remedy that prevents or slows the course of an illness or
         disease; "the doctor recommended several preventatives"
         [syn: preventive, preventative, prophylactic]
    2: contraceptive device consisting of a sheath of thin rubber or
       latex that is worn over the penis during intercourse [syn:
       condom, rubber, safety, safe, prophylactic]

Moby Thesaurus II by Grady Ward, 1.0:

121 Moby Thesaurus words for "prophylactic":
   IUD, aegis, antacid, antidote, antiseptic, arm guard, aseptic,
   backstop, bag, birth control device, boiled, buffer, bulwark,
   bumper, condom, contraceptive, contraceptive foam, copyright,
   counteractant, counteractive, counteragent, counterirritant,
   covering, crash helmet, cushion, custodial, dashboard,
   decontaminated, defensive, deterrent, deterring, diaphragm,
   discouraging, disinfected, dodger, face mask, fender, finger guard,
   foot guard, forbidding, forestalling, fostering, fuse, goggles,
   governor, guard, guardian, guarding, guardrail, hand guard,
   handrail, hard hat, helmet, hygienic, immunizing, insulation,
   interlock, intrauterine device, knee guard, knuckle guard,
   laminated glass, life preserver, lifeline, lightning conductor,
   lightning rod, mask, mudguard, neutralizer, nose guard, nullifier,
   offset, oral contraceptive, pad, padding, palladium, parental,
   pasteurized, patent, pessary, pilot, preclusive, preventative,
   preventive, prohibitive, protecting, protective,
   protective clothing, protective umbrella, remedy, rubber,
   safeguard, safeguarding, safety, safety glass, safety plug,
   safety rail, safety shoes, safety switch, safety valve, sanitary,
   screen, screening, seat belt, sheltering, shield, shielding,
   shin guard, skin, spermicidal jelly, spermicide, sterile,
   sterilized, sun helmet, the pill, tutelary, umbrella, uninfected,
   vigilant, watchful, windscreen, windshield