A contusion is a bruise: a place that has been hit hard enough for blood to pool under the skin. The skin often stays closed. This is the noun doctors put on reports and in textbooks; everyone else says bruise. The word itself does not tell you how bad it is — a contusion can be a small mark on the arm, or a serious blow to the chest or head. The doctor has to say that separately. Though actually, people sometimes mix this up with concussion, which is a brain injury from a knock to the head. A contusion is the bruise in the flesh; a concussion is about the brain being shaken. You can have one, the other, or sadly both. In careful medical talk, a 'pulmonary contusion' is bruising of the lung, which is much more serious than a bruise you can see.