Bone marrow aspiration: The removal of a small amount of bone marrow (usually from the hip) through a needle. The needle is placed through the top layer of bone and a liquid sample containing bone marrow cells is drawn into a syringe. The sample is then sent for cytologic examination. Bone marrow aspiration is done to diagnose and follow the progress of various conditions, including anemia and cancer, and to obtain marrow for transplantation.