A scar is the visible part of an injury to the dermis after tissue is repaired, following an incision made during an operation, or following an injury.
Scarring is an integral part of the healing process. Apart from very minor injuries, each injury (after an accident, illness, or surgery) causes a more or less significant scar. Scar tissue is not the same as the tissue it replaces and is usually of lower functional quality. For example, skin scars are more sensitive to ultraviolet radiation, sweat glands and hair follicles do not develop under the scar; the formation of scars following a myocardial infarction causes a loss of power of the heart muscle and increases the risk of cardiac arrest.
However, some tissues can heal without structural or functional deterioration.