A substance given to stimulate the body's production of antibodies and provide immunity against a disease without causing the disease itself in the treatment, prepared from the agent that causes the disease (or a related, also effective, but safer disease), or a synthetic substitute.
"The doctor administered the new flu vaccine to protect her child from contracting influenza while avoiding any risk of severe illness."
In plain English: A vaccine is medicine that trains your body to fight off specific diseases before you actually get sick from them.
"The doctor gave her child the flu vaccine to protect against illness this winter."
Usage: A vaccine is an injection or pill designed to train your immune system to fight specific diseases before you get sick. Use this word for preventative biological preparations, not as a synonym for general medicine or treatment after symptoms appear.
Learned borrowing from Latin vaccīnus ("of or derived from a cow"), from vacca ("cow (female cattle)") + -īnus (suffix meaning 'of or pertaining to' forming adjectives). Sense 1 refers to the early use of the cowpox virus as a vaccination against smallpox: see New Latin variolae vaccīnae ("cowpox", plural, literally "infectious diseases of cattle causing pustules"), coined by the British physician and scientist Edward Jenner (1749-1823).