Gastrointestinal (GI) bleeding refers to any bleeding that occurs within the gastrointestinal tract, which includes the mouth, esophagus, stomach, small intestine, large intestine, and rectum. It can manifest as either visible blood in vomit or stool or as occult (hidden) blood detected through diagnostic tests. GI bleeding can result from various causes, such as ulcers, inflammation, polyps, or cancer.