What is the definition or description of: bone marrow?

See below: Bone marrow is the flexible tissue found in the interior of bones. In humans, red blood cells are produced in the heads of long bones, in a process known as hematopoesis. On average, bone marrow constitutes 4% of the total body mass of humans; in an adult weighing 65 kilograms (140 lb), bone marrow accounts for approximately 2.6 kilograms (5.7 lb).