A cavity is a hole or hollow space inside something solid. Dentists use the word to describe holes that form in teeth when bacteria eat away at the hard outer surface. When you eat sugary foods and don't brush regularly, tiny organisms called bacteria produce acid that slowly dissolves the tooth's protective coating, creating a cavity. If left untreated, the hole gets bigger and deeper, eventually causing pain and requiring a filling to repair the damage.

Cavities appear in many contexts beyond dentistry. Scientists talk about the chest cavity (the space inside your ribcage where your heart and lungs sit) or the nasal cavity (the hollow space behind your nose). Architects might design a building with a wall cavity for insulation.

When you hear someone say they have a cavity, they usually mean the dental kind. Cavities are almost completely preventable with regular brushing, flossing, and limiting sugary snacks. Once you understand that a cavity is essentially tooth decay that has eaten a hole in your tooth, you understand why dentists care so much about prevention.