Filling is the substance or material placed inside something else. When you bite into a jelly donut, the sweet jelly is the filling. When a dentist repairs a cavity in your tooth, they put in a filling made of metal or ceramic to patch the hole. A pillow's filling might be soft feathers, foam, or polyester that makes it comfortable.

The word also describes food that makes you feel full and satisfied. A bowl of oatmeal is more filling than a handful of crackers because it sticks with you longer and stops your hunger. Dense foods like potatoes, rice, and beans are particularly filling because they take time to digest.

Filling can be a noun (the thing that goes inside) or an adjective (describing something that satisfies your hunger). A sandwich with peanut butter and banana might have a tasty filling and also be quite filling, keeping you energized until dinner. The connection between these meanings makes sense: both involve something substantial that fills a space, whether that space is inside a pastry or inside your stomach.