A bruise, also known as a contusion, is a type of hematoma of tissue, the most common cause being capillaries damaged by trauma, causing localized bleeding that extravasates into the surrounding interstitial tissues. Most bruises occur close enough to the epidermis such that the bleeding causes a visible discoloration. The bruise then remains visible until the blood is either absorbed by tissues or cleared by immune system action. Bruises which do not blanch under pressure can involve capillaries at the level of skin, subcutaneous tissue, muscle, or bone.

Bruises are not to be confused with other similar-looking lesions. Such lesions include (1) petechia (less than 3 mm (0.12 in), resulting from numerous and diverse etiologies such as adverse reactions from medications such as warfarin, straining, asphyxiation, platelet disorders and diseases such as cytomegalovirus); and (2) purpura (3–10 mm (0.12–0.39 in)), classified as palpable purpura or non-palpable purpura and indicating various pathologic conditions such as thrombocytopenia. Additionally, (3) although many terminology schemas treat an ecchymosis (plural, ecchymoses) (size, more than 1 cm (0.39 in)) as synonymous with a bruise, in some other schemas, an ecchymosis is differentiated by its remoteness from the source and cause of bleeding, with blood dissecting through tissue planes and settling in an area remote from the site of trauma or even nontraumatic pathology, such as in periorbital ecchymosis ("raccoon eyes"), arising from a basilar skull fracture or from a neuroblastoma.

As a type of hematoma, a bruise is always caused by internal bleeding into the interstitial tissues which does not break through the skin, usually initiated by blunt trauma, which causes damage through physical compression and deceleration forces. Trauma sufficient to cause bruising can occur from a wide variety of situations including accidents, falls, and surgeries. Disease states such as insufficient or malfunctioning platelets, other coagulation deficiencies, or vascular disorders, such as venous blockage associated with severe allergies can lead to the formation of purpura which is not to be confused with trauma-related bruising/contusion. If the trauma is sufficient to break the skin and allow blood to escape the interstitial tissues, the injury is not a bruise but bleeding, a different variety of hemorrhage. Such injuries may be accompanied by bruising elsewhere.