pro·phy·lac·tic 
(prō′fə-lăktĭk, prŏf′ə-)
(prō′fə-lăktĭk, prŏf′ə-)adj.
Acting to defend against or prevent something, especially disease; protective.
n.
1. A prophylactic agent, device, or measure, such as a vaccine or drug.
2. A contraceptive device, especially a condom.
[French prophylactique, from Greek prophulaktikos, from prophulassein, to take precautions against : pro-, before; see PRO-2 + phulassein, to protect (from phulax, guard).]
pro′phy·lacti·cal·ly adv.