1. Protecting a person, or animal, against a particular disease by introducing a special substance into their bodies, usually by injection.
2. A vaccination or inoculation, a method of stimulating resistance in the human body to specific diseases using microorganisms (bacteria or viruses) that have been modified or killed: These treated immunizations of microorganisms do not cause the disease, but rather trigger the body's immune system to build a defense mechanism that continuously guards against such diseases.