A blister agent, or vesicant, is a chemical compound that causes severe pain on the skin, in the eyes, and in the mucus membranes. They can cause severe chemical burns, which then cause painful water blisters, on both the outside and the inside of the body. (This is why these chemicals are called "blister agents.") Throughout history, blister agents have been used for chemical warfare.
Some blister agents are used to treat medical problems - for example, to remove warts. However, they must be used very carefully. If a person accidentally gets even a little bit of the chemical in their mouth, they can die.