In the picture framing

Picture frame

A picture frame is a decorative edging for a picture, such as a painting or photograph, intended to enhance it, make it easier to display, or protect it.-Construction:...

 industry, a mat (or mount in British English

British English

British English, or English , is the broad term used to distinguish the forms of the English language used in the United Kingdom from forms used elsewhere...

) is a thin, flat piece of paper

Paper

Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....

-based material included within a picture frame, which serves as additional decoration and to perform several other, more practical functions, such as separating the art from the glass. Putting mats in a frame is called matting, a term which can also usually be used interchangeably with mat.
The picture-framing mat is most commonly known by laymen for its use as additional decoration to enhance the look of a framed piece, sometimes in conjunction with a fillet

Fillet (picture framing)

In the picture framing industry, a fillet is a small piece of moulding which fits inside a larger frame or, typically, underneath or in between matting, used for decorative purposes...

 or more rarely, liners made of wooden moulding

Molding (decorative)

Molding or moulding is a strip of material with various profiles used to cover transitions between surfaces or for decoration. It is traditionally made from solid milled wood or plaster but may be made from plastic or reformed wood...

 with a cloth surface.