Longevity escape velocity: the point at which life-extending technologies improve at a fast enough rate to allow new life-extension technologies to be developed before death. If we achieve this as a species, we (or at least some of us) can become effectively “functionally immortal” (mmm, immortality) – we won't have to die from disease or debilitation, although we can always fall prey to accident or foul-play.
Life expectancy increases slightly every year as treatment strategies and technologies improve. At present, more than one year of research is required for each additional year of expected life. Longevity escape velocity occurs when this ratio reverses, so that life expectancy increases faster than one year per one year of research, as long as that rate of advance is sustainable.