What is the cardboard inside a picture frame called?

The cardboard inside a picture frame is called a mat board. Picture frame mats are thick & add a professional touch to your framed images.
The thick material inside a picture frame is called a picture frame mat. It's typically sturdy like cardboard, but it isn't actually cardboard. Crafted out of sturdy mat board material, picture frame mats are an easy way to highlight a framed photo, art print, or map. They are available

No One Cares About My Framed NFT Art

23 Unexpected Items to Put in a Picture Frame

Why You Need a Photo Booth at A Brand Activation

Custom Matboards

GAN 12 3x3 Magnetic (2 Versions) SpeedCubeShop, 41% OFF

Framing Vintage Photos in old picture frames - The Honeycomb Home

Picture Frames Parts Names - Photo Frames and Picture Frames Online Store

Matboard and More

terminology - Name of fastening metal pieces holding the cardboard back to a picture frame? - English Language & Usage Stack Exchange

The Frame Room Frame & Design Blog - How to Store Picture Frames Properly

What's inside a picture frame? - Photo Frames and Picture Frames Online Store

What is the paper frame around a picture called?

Sand Sand Wave Pattern On The Surface Of The Solid Orange, 60% OFF

DIY Cardboard Picture Frame Mother's Day Craft - Happy Toddler Playtime