Which of the following best describes a blister?

A blister is best described as a vesicle, which is a small fluid-filled sac that forms within the upper layers of the skin. It occurs when the skin is damaged, often due to friction, heat, or other irritation. The body's response to injury is to collect tissue fluid or serum in order to protect the underlying layers of the skin and promote healing. This collection of fluid forms a bubble-like structure, which is what is commonly recognized as a blister.

In distinguishing this from the other options, a blister is not a type of skin infection, nor is it specifically associated with pus, which would indicate infection rather than a fluid-filled vesicle. Additionally, while a mole or skin tag may also appear on the skin, they are distinct in nature and appearance, not related to the definition of a blister.