What is the definition or description of: fecal occult blood?

FOBT: Fecal occult blood testing is a tool you use at home to detect tiny amounts of blood in your stool. This may suggest that there is a polyp or cancer as the cause of the bleeding. Although the vast majority of positive tests for occult blood do not have a cancer, a positive result means you should have another test (such as colonoscopy) to investigate this further.