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 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 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, although many terminology schemas treat an ecchymosis (plural, ecchymoses) (over 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.

Bruises often induce pain immediately after the trauma that results in their formation, but small bruises are not normally dangerous alone. Sometimes bruises can be serious, leading to other more life-threatening forms of hematoma, such as when associated with serious injuries, including fractures and more severe internal bleeding. The likelihood and severity of bruising depends on many factors, including type and healthiness of affected tissues. Minor bruises may be easily recognized in people with light skin color by characteristic blue or purple appearance (idiomatically described as "black and blue") in the days following the injury.

Hematomas can be subdivided by size. By definition, ecchymoses are 1 centimetres in size or larger, and are therefore larger than petechiae (less than 3 millimetres in diameter) or purpura (3 to 10 millimetres in diameter). Ecchymoses also have a more diffuse border than other purpura. A broader definition of ecchymosis is the escape of blood into the tissues from ruptured blood vessels. The term also applies to the subcutaneous discoloration resulting from seepage of blood within the injured tissue.

Bruise colors vary from red, blue, or almost black, depending on the severity of broken capillaries or blood vessels within the bruise site. Broken venules or arterioles often result in a deep blue or dark red bruise, respectively. Darker colored bruises may result from a more severe bleeding from both blood vessels. Older bruises may appear yellow, green or brown.

There are many causes of subcutaneous hematomas including ecchymoses. Coagulopathies such as hemophilia A may cause ecchymosis formation in children. The medication betamethasone can have the adverse effect of causing ecchymosis.

The presence of bruises may be seen in patients with platelet or coagulation disorders, or those who are being treated with an anticoagulant. Unexplained bruising may be a warning sign of child abuse, domestic abuse, hard blows during fighting sports, shot by rubber bullets during protests, hit by falling objects/stuck inside rubble during earthquakes, hit by hailstones/falling volcanic debris caused by volcanic eruptions with alert level IV–V ("warning") and/or serious medical problems such as leukemia or meningoccocal infection. Unexplained bruising can also indicate internal bleeding or certain types of cancer. Long-term glucocorticoid therapy can cause easy bruising. Bruising present around the navel (belly button) with severe abdominal pain suggests acute pancreatitis. Connective tissue disorders such as Ehlers–Danlos syndrome may cause relatively easy or spontaneous bruising depending on the severity. Spontaneous bruising or bruising with minimal trauma in the absence of other explanations and together with other minor or major criteria suggestive of vascular Ehlers–Danlos Syndrome (vEDS) suggests genetic testing for the condition.