Biological immortality can be defined as the absence of a sustained increase in rate of mortality as a function of age. A cell or organism that does not experience, or at some point ceases experiencing, aging is biologically immortal.
Examples of biologically immortal cells include germ cells, embryonic stem cells, and cancer cells. There is no limit to the number of times these cells may divide, as opposed to mortal cells which are limited to a set number of divisions. Some rudimentary organisms are biologically immortal.
It is the theorized by some that when it is discovered exactly what allows some cells to be biologically immortal that it will be possible to genetically alter other cells to have the same capability (most believe telomere length is involved). It is further theorized that it will eventually be possible to genetically engineer all cells in the human body to have this capability by employing gene therapy and thereby stop or reverse ageing, effectively making the entire organism biologically immortal. Scientists pursuing human biological immortality are commonly referred to as biogerontologists.