Prophylaxis originates from a Greek term which literally means “to guard or to prevent beforehand.” In medical terms, prophylaxis is used to refer to a group of procedures which serve to prevent a disease or an illness, as opposed to having to cure such a condition.
There are two general kinds of prophylaxis: primary prophylaxis and secondary prophylaxis. Primary prophylaxis is used to refer to procedures that prevent the development of a disease - the patient is relatively healthy when the procedures are done. Secondary prophylaxis is used to refer to procedures that prevent the worsening of a disease that has already developed.
Perhaps the most commonly known use of prophylaxis is in the dental context. Oral prophylaxis is basically the cleaning of the teeth and gums. This falls under primary prophylaxis, as it prevents oral problems such as tooth decay, gum diseases, and the like. The administering of vaccines are also considered prophylaxis as vaccines are used to prevent specific kinds of illnesses.
You may be familiar with the use of the term prophylactics to refer to condoms. This is not a mere coincidence, as condoms do prevent pregnancy (although it is not really an illness) and sexually transmitted diseases. Other examples of prophylaxis include antibiotics (as in cases where individuals are exposed to substances such as anthrax), antiretroviral drugs (for persons exposed to HIV/AIDS virus), and radiation (for cancer patients).
As one can surmise, prophylaxis is a very broad term that can apply to practically any field of medicine.