Hematemesis, or vomiting blood, occurs when there is bleeding in the upper gastrointestinal tract, typically originating from the esophagus, stomach, or upper small intestine. It presents as the regurgitation of bright red or coffee ground-like material, indicating the presence of blood. Hematemesis can be caused by various conditions, including peptic ulcers, esophageal varices, gastritis, or Mallory-Weiss tears. Prompt medical evaluation is essential to identify the source of bleeding and initiate appropriate treatment.