Biological immortality can be defined as the absence of a sustained increase in rate of mortality as a function of chronological age. A cell or organism that does not experience, or at some future point will cease aging, is biologically immortal. However this definition of immortality was challenged in the new "Handbook of the Biology of Aging" , because the increase in rate of mortality as a function of chronological age may be negligible at extremely old ages (late-life mortality plateau), yet the mortality rates are particularly high at old ages.

There is no organism or individual cell that is literally immortal. Any "immortal" cell or organism can be killed by physical destruction.

Biologists have chosen the word immortal to designate cells that are not limited by the Hayflick limit (where cells no longer divide because of DNA damage or shortened telomeres). The term immortalization was first applied to cancer cells that developed the telomere-lengthening enzyme telomerase, and thereby avoided apoptosis (programmed cell death). In terms of multi-cellular organisms, immortality may not be a desirable condition, as the main controls over cancer are the apoptotic mechanisms.

Bacteria can be said to be biologically immortal, but only as a colony. An individual bacterium can easily die. The two daughter bacteria resulting from cell division of a parent bacterium can be regarded as unique individuals or as members of a biologically "immortal" colony. The two daughter cells can be regarded as "rejuvenated" copies of the parent cell because damaged macromolecules have been split between the two cells and diluted. In the same way stem cells and gametes can be regarded as "immortal". Similarly, Hydra, lobsters and a number of other species are regarded as never aging because they are composed entirely of cells that never cease dividing.

Some life extensionists, such as those who practice cryonics, have the hope that humans may someday become biologically immortal. This would obviously not be the same as being literally immortal because people can always be murdered or die in accidents.

Nanotechnology, and specifically of nanomedicine, have recently increased awareness of the possibilities for biological immortality. A study published in Physiological and Biochemical Zoology in 2005 indicates that biological immortality may exist in humans at a late stage in life: "the exponential increase in age-specific death rate seemed to slow down considerably, if not cease." ^*

The biogerontologist Aubrey de Grey of Cambridge University has proposed that damage to macromolecules, cells, tissues and organs could be repaired by advanced biotechnology. He calls his project SENS (Strategies for Engineered Negligible Senescence). Dr. de Grey has created the Methuselah Mouse Prize to award money to researchers who can extend the lifespan of, or rejuvenate mice.

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