A callus (pl.: calluses) is an area of thickened and sometimes hardened skin that forms as a response to repeated friction, pressure, or other irritation. Since repeated contact is required, calluses are most often found on the feet and hands, but they may occur anywhere on the skin. Some degree of callus, such as on the bottom of the foot, is normal.

Calluses are generally not harmful and help prevent blisters, as well as offering protection. However, excessive formation may sometimes lead to other problems, such as a skin ulceration or infection, or cause the affected person to try to offload the affected painful area, which can place excessive stress on the asymptomatic side.

Rubbing that is too frequent or forceful will cause blisters, as opposed to calluses, to form.

Normally, a callus will form on any part of the skin exposed to excess friction over a long period of time. Activities that are known for causing calluses include construction work and craftwork, the arts, food preparation, many sports and physical activities, and fashion choices like wearing high heels.

Although calluses can occur anywhere on the body as a reaction to moderate, constant "grinding" pressure, they are most often found on the foot (where the most pressure and friction are applied). On the feet (arguably the source of the most problematic calluses), they typically form on the metatarsal-phalangeal joint area ("balls of the foot"), heels and small toes due to the compression applied by tightly fitting shoes.

Biologically, calluses are formed by the accumulation of terminally differentiated keratinocytes in the outermost layer of skin. Though the cells of calluses are dead, they are quite resistant to mechanical and chemical damage due to extensive networks of cross-linked proteins and hydrophobic keratin intermediate filaments containing many disulfide bonds. It is the natural reaction of the palmar or plantar skin. Too much friction occurring too fast for the skin to develop a protective callus will cause a blister or abrasion instead.

Sometimes a callus occurs where there is no rubbing or pressure. These hyperkeratoses can have a variety of causes. Some toxic materials, such as arsenic, can cause thick palms and soles. Some diseases, such as syphilis, can cause thickening of the palms and soles as well as pinpoint hyperkeratoses. There is a benign condition called keratosis palmaris et plantaris, which produces corns in the creases of the fingers and non-weight bearing spaces of the feet. Some of this may be caused by actinic keratosis, which occurs due to overexposure to sun or with age and hormonal shifts.

A corn (or clavus, plural clavi) is a cone-shaped callus that penetrates into the dermis, usually on the feet or hands. Corns may form due to chronic pressure or rubbing at a pressure point (in this skin over a bone), or due to scar tissue from a healing wound creating pressure in a weight-bearing area such as the sole of the foot. If there is constant stimulation of the tissue producing the corns, even after the corn is surgically removed, the skin may continue to grow as a corn.