necrobiosis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The natural death of cells or tissues through aging, as distinguished from necrosis or pathological death.

from The Century Dictionary.

• noun In pathology, degenerative progress toward and ending in the death of a portion of tissue.
• noun In biology, the doctrine or opinion that living beings may arise from decomposing organic matter: contrasted with the opinion that they may be spontaneously generated from inorganic matter.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Biol. & Med.) The death of a part of an organism by molecular disintegration and without loss of continuity, as in the processes of degeneration and atrophy; a normal dying out of cells in a tissue, in contrast to necrosis.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun pathology The normal death of cells or tissues because of age-related changes.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (physiology) the normal degeneration and death of living cells (as in various epithelial cells)