A blister is a bubble of fluid that forms under your skin, usually caused by friction or heat. When your shoe rubs the same spot on your heel over and over, the top layer of skin separates from the layers underneath, and clear liquid fills the gap. That painful bubble is a blister.

Blisters often appear on hands and feet. You might get one from raking leaves for an hour without gloves, or from breaking in new shoes on a long walk. The fluid inside acts like a cushion, protecting the raw skin underneath while it heals.

Heat can also cause blisters. If you accidentally touch a hot pan, a blister might form on your finger within hours. Some illnesses cause blisters too, like chickenpox, which creates small, itchy blisters on the body.

As a verb, blister means to form blisters. The word can also describe paint or other surfaces that bubble up. When old paint on a fence starts blistering, it forms little bubbles where it's pulling away from the wood underneath, similar to how skin blisters form.