Biological Immortality

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). But even though the rate of mortality ceases to increase in old age, those rates are very high (e.g., 50% chance of surviving another year at age 110 or 115 years of age).
There is no known organism or individual cell that is inviolably immortal. Any life enjoying biological immortality can die if exposed to a toxic environment, or otherwise killed or destroyed