Prophylactic: A preventive measure. The word comes from the Greek for "an advance guard," an apt term for a measure taken to fend off a disease or another unwanted consequence.
A prophylactic is a medication or a treatment designed and used to prevent a disease from occurring. For example, prophylactic antibiotics may be used after a bout of rheumatic fever to prevent the subsequent development of Sydenham's chorea.
A prophylactic is also a drug or device, particularly a condom, for preventing pregnancy.