1Originally: abnormal proliferation of white blood cell (granulocyte) precursors; an instance of this. Later also (chiefly with distinguishing word): abnormal proliferation of erythroid precursors or megakaryocytes.
2Degenerative disease of the spinal cord; specifically (in full "funicular myelosis") degeneration of the posterior and lateral columns, sometimes associated with pernicious anaemia.
Mid 19th century (in an earlier sense). From myelo- + -osis.