Longevity (as it applies to supplementation) is the concept of promoting or preserving youth and vitality via delaying the inherent human aging process. Research into this topic is young and speculative, but generally centers around autophagy.
Longevity is a term used to refer to a preservation of vitality and physical/mental robustness over a prolonged period of time, perhaps exceeding the average lifespan of other humans. The pursuit for longevity tends to look for both chronological enhancement (life extension) and either preserving or enhancing function during this chronological enhancement (vitality) and attempts to capitalize on both 'adding years to life' as well as 'adding life to years'.
Longevity research begins with experimentation on yeast, nematodes (C. Elegans), and fruit flies (drosophilia) due to their short life cycles. Fluctuations in the human lifespan would take an impractical 90 years to test; C. Elegans' 90-day lifespan serves as a convenient alternative for initial longevity trials. Once mechanisms are established in these shorter research models, studies are conducted in mammalian species such as mice and rats. Human studies are nonexistent for practical reasons.
Additionally, caloric restriction (40-50% of standard caloric intake) appears to reliably produce life extension in all tested non-human subjects and case studies (anecdotes) of humans on caloric restriction suggest it delays the aging phenotype; due to the high possibility that caloric restriction is able to enhance lifespan, many mechanisms and phenomena are considered to be 'caloric restriction related' or 'caloric restriction independent'.