Medical Definition of NECROSIS. : death of living tissue; specifically: death of a portion of tissue differentially affected by local injury (as loss of blood supply, corrosion, burning, or the local lesion of a disease)—compare necrobiosis.edical Definition of NECROSIS. : death of living tissue; specifically: death of a portion of tissue differentially affected by local injury (as loss of blood supply, corrosion, burning, or the local lesion of a disease)—compare necrobiosis.