A tangle of fur that has knotted together into a dense, compressed mass, typically forming close to the skin. Mats are caused by a combination of dead coat that wasn't brushed out, friction from movement, and moisture. They most commonly form in high-friction areas: armpits, behind ears, under collars, and in groin areas.
Severe matting can be a welfare issue — tight mats pull on the skin, restrict blood flow, hide parasites and skin infections, and cause significant pain. Once a mat has tightened against the skin, humane removal often requires a full shave-down rather than brushing out.
Never try to brush out a tight mat that is close to the skin — you risk tearing the skin. Use a mat splitter to divide the mat into smaller sections first, then work each section with a dematting comb.