matting
A stiff border around a picture inside its frame.
Matting is a border or frame, usually made of stiff cardboard or similar material, that surrounds a picture or photograph before it goes into a frame. When you visit an art museum, you'll often see paintings and photographs displayed with a wide border of white, cream, or colored material between the artwork and the frame itself. That border is the matting.
Matting serves several purposes. It creates visual breathing room around the artwork, drawing your eye toward the picture while separating it from the frame. It also protects the art by keeping it from touching the glass directly. Professional matting is cut with beveled edges (angled rather than straight) for a polished look.
The word can also describe the process of adding this border: an artist might spend time matting their drawings before a school art show. Custom matting can be expensive because it requires precise cutting, which is why many people buy pre-cut mats in standard sizes.
The word matting has an unrelated meaning too: tangled, messy clumps, especially in hair or fur. When a dog's coat isn't brushed regularly, it starts matting into uncomfortable knots that need to be carefully combed out.