means a substance that trains your immune system to recognize and fight a specific disease, usually given as a shot to prevent illness before exposure.

from From Latin vaccinus, "of or from cows" (vacca, "cow"). The cow earns its place honestly: in the late 18th century, Edward Jenner noticed that milkmaids who had caught cowpox seemed spared from the far deadlier smallpox. He used material from cowpox sores to protect against smallpox, and the technique took the cow's name with it. Louis Pasteur later broadened the word to cover all such preventives, in tribute to Jenner — so every modern vaccine still quietly tips its hat to a dairy cow.