The phenotypic observation that death has occurred in groups of cells or in one or more tissues due to external factors, such as infection, toxins, mechanical trauma, loss of blood supply and other injuries (e.g. corrosion or burning). At the cellular level, morphological changes following necrosis include a translucent cytoplasm, swelling of organelles, modifications of the nucleus and increased cell volume (oncosis), culminating in the disruption of the plasma membrane and subsequent loss of intracellular contents. Necrotic cells do not fragment into discrete corpses as their apoptotic counterparts do. Moreover, their nuclei remain intact and can aggregate and accumulate in necrotic tissues. The phenotype is Experimental Factor Ontology id EFO_0009426 (necrosis). Also known as: tissue necrosis.