A hematoma, also spelled haematoma, or blood suffusion is a localized bleeding outside of blood vessels, due to either disease or trauma including injury or surgery and may involve blood continuing to seep from broken capillaries. A hematoma is benign and is initially in liquid form spread among the tissues including in sacs between tissues where it may coagulate and solidify before blood is reabsorbed into blood vessels. An ecchymosis is a hematoma of the skin larger than 10 mm.

They may occur among and or within many areas such as skin and other organs, connective tissues, bone, joints and muscle.

A collection of blood (or even a hemorrhage) may be aggravated by anticoagulant medication (blood thinner). Blood seepage and collection of blood may occur if heparin is given via an intramuscular route; to avoid this, heparin must be given intravenously or subcutaneously.