Which of the following is defined as a fluid- or gas-filled bubble in the skin?

A cyst is defined as a fluid- or gas-filled sac that can develop in various tissues of the body, including the skin. Cysts are typically encapsulated, meaning they are surrounded by a membrane, and can vary in size. When they occur in the skin, they may appear as raised bumps or lumps. The fluid or gas inside can create pressure, sometimes leading to discomfort or pain, especially if the cyst becomes infected or inflamed.

The other options are distinct dermatological terms that do not fit the same definition. A furuncle, commonly known as a boil, is an infected hair follicle filled with pus, not a fluid-filled cavity. Beau's lines are transverse lines or grooves on the nails, indicating a disruption in nail growth, while a lunula refers to the visible part of the root of the nail, not a bubble or cystic structure in the skin.