Stem cell transplantation: The use of stem cells as a treatment for cancer or other illness. The stem cells are removed (or obtained from a donor) first. Before the transplant is done, the patient receives high-dose chemotherapy and/or radiation therapy to destroy diseased cells. Then the stem cells are returned to the patient, where they can produce new blood and immune cells and replace the cells destroyed by the treatment. The stem cell preparation is infused into a vein and, once in the bloodstream, the stem cells migrate to the bone marrow space.