Webster1913 is entirely correct. Even today, prophylactic is commonly used to mean any medicine that is given to prevent disease. It is also used for operations (for example, a prophylactic oophorectomy or a prophylactic mastectomy), implants (for example, implanting a defibrillator for someone at risk for cardiac arrest), and can even be used to refer to things like the dentist cleaning your teeth (to prevent cavities). Secondary prophylaxis refers to a prophylaxis used to prevent the reoccurrence or relapse of a disease.

But wait! I just switched words on you! What is this prophylaxis thing? Well, no one is quite clear on what exactly the difference between the two is; they both come from the same root, both mean the same thing, and in many cases can be used interchangeably. Prophylactic can be both a noun and an adjective, and is generally used to refer to specific medications. Prophylaxis is only used as a noun, and seems to be used more often when you are referring to more general preventative medicine, without specifying the specific medicine involved, although that is only a general trend (if even that). It seems as though many people are using prophylaxis as if it were the plural of prophylactic, but it is singular. The plural of prophylaxis is prophylaxes; the plural of prophylactic is prophylactics.

Prophylactic is also a common polite euphemism for a condom. It first came into usage in 1943, replacing the earlier euphemism 'preventive' (which you may occasionally still see in use). Both are purely descriptive and non-specific terms that could (hypothetically) be 'prescribed' by doctors without causing outrage in the community. The word condom predates both these euphemisms by at least a century.

From the Greek prophylaktikos, meaning 'precautionary'. Pronounced 'pro-fuh-lak-tik.

References: Dictionary.reference.com
Webster's Third New International Dictionary, 1981