A term coined by Edward Jenner for the process of administering a weakened form of a disease to patients as a means of giving them immunity to a more serious form of the disease. The word comes from the Latin for cow, in honor of the cowpox virus--a relatively benign virus that was discovered to have the side effect of conferring immunity to smallpox, a deadly and highly contagious disease.
Since Jenner's time vaccination has become widespread and applicable to diseases other than smallpox. Generically the process of protecting against an infectious disease by "priming" the immune system with material, the immunogen, designed to stimulate an immune response to the infectious agent is known as immunization. Vaccination is used where the immunogen is itself a living infectious agent, normally either a closely related bacterial species (as with smallpox and cowpox), or by using a strain weakened by some process. In this case the immunogen is called a vaccine.
In its greatest triumphs, vaccination campaigns have eliminated smallpox throughout the world and restricted polio to some war-torn countries in Africa where health care access is difficult. Vaccines have made other once common childhood diseases rare, as mumps, measles, etc.