Longevity, even exaggerated with terms such as immortality and everlasting life, is the ability shared by some - possibly all - witches to live for a considerably longer period than the normal life expectancy of the average human being.
There are several ways to supernaturally lengthen life expectancy ranging from altered physiology which seems to be a distinctive feature of all witches (thus having a different physiological predisposition than the average human, including an increased lifespan as well as increased physical endurance) to more complex and elaborate rituals of blooming that require the use of blood sacrifices in order to also remain eternally young and healthy. The so-called eternal life has consequences that depend primarily on the ritual used to obtain it, or the mingling of different rituals to achieve certain effects. A witch who lives eternally is not necessarily indestructible. Countess Von Marburg, for example, can be killed in her various incarnations, even with crude methods like stabbing, yet rises again every time since her longevity is guaranteed by the spell placed on her relic.
Countess Von Marburg is one of the oldest witches on Earth, if not the oldest still alive. While her beauty and youth are kept by constantly bathing in human blood, she is able to live forever due to the fact that she has preserved a vital spark in her original body, kept as a sacred relic in a shrine. This very relic is the source of her everlasting life because while the physical body she took at every incarnation can be killed, only hurting the original body can cause her serious harm, otherwise she will resurrect through magical feats not yet revealed.
Sebastian Von Marburg is at least two centuries years old according to promotional biography released by the showrunners.
Widdershins, by their own admission, have been around for a thousand years. However, their bodies are covered with sores and they look extremely old. Has not yet been revealed how the Widdershins have survived for so long, but surely they not resort to blooming spells to preserve youth and beauty, unlike Countess Von Marburg and her son Sebastian.
The Hags, emissaries of the Essex Hive, also appear to be considerable old.
Although not explicitly revealed in the show, it is assumed that Deities such as the Devil or the Great Spirit possess this skill, or even that they are immortal in the literal sense of the term as they are supreme beings of power.
Regarding Immortality, a special mention should be made about familiars. These creatures can be killed, dismembered and bled to give blood during sacrifices but do not seem to suffer any permanent damage. In fact, even if they were never seen to resurrect on screen, they simply reappear alive and well after scenes in which they were brutally killed, without an actual explanation.
Longevity may lie in the fact of never suffering any mortal harm or from the fact that the witch in question is able to resurrect over and over again.