Immortality

Immortality is the ability to live forever or avoid permanent death, either due to inherent abilities or use of external methods to prolong one's life indefinitely. Various life states throughout The Sims (series) possess this ability, and multiple methods exist to achieve immortality.

Immortal life states
There are many life states throughout the series which are inherently immortal, and possess the capability to live forever unless killed through unnatural means.

The Sims The Sims 2 The Sims 3 The Sims 4
 * Ghosts cannot be killed due to already being dead, although they can be revived and killed again.
 * Ghosts are immortal in essentially the same way as their counterparts from The Sims.
 * In The Sims 2: University, Zombies are unable to age past the Adult life stage, and therefore possess the potential to "live" forever.
 * In The Sims 2: Nightlife, Vampires, much like zombies, cannot age, although they can be killed by other means, notably their vulnerability to sunlight.
 * In The Sims 2: Open for Business, Servos cannot age, and can only be killed by a falling satellite, being eaten by the Cowplant, or running with scissors.
 * Much like in the previous two Sims games, Ghosts are still essentially immortal, although they can be permanently removed from the game if their tombstone is deleted or they are made playable and choose to Return to Netherworld.
 * In The Sims 3: Late Night, while still possessing greatly augmented lifespans, vampires are no longer immortal and can die of old age, although if The Sims 3: Supernatural is installed they can acquire the Immortal lifetime reward to make them completely ageless and immune to the sunlight.
 * In The Sims 3: Into the Future, Plumbots, much like Servos, cannot die of old age and can only be killed by falling meteor, although some store items such as the Cowplant or the Door of Life And Death are capable of killing Plumbots.
 * Ghosts remain immortal, but can be killed if they are revived first.

Methods of becoming immortal
The Sims 2
 * Sims with a high enough Aspiration Meter can drink the Elixir of Life and continously purchase resupplies of the elixir when they run out, effectively rendering themselves ageless. However, this method not only requires the Sim to maintain a high Aspiration Meter, but also requires the Sim or another Sim in the household to be able to afford replacement elixirs each time one runs out.
 * Similarly to the above method, if The Sims 2: University is installed, a Sim can use the elixir produced by the Cowplant to achieve the same effect. This has the benefit of not requiring a certain aspiration level but comes with the drawback of requiring other Sims to be sacrificed to the Cowplant on a regular basis.
 * If The Sims 2: University is installed, Sims can be brought back to life through use of the Resurrect-O-Nomitron by another Sim. Since this process can be repeated an indefinite number of times, any household can avoid permanent death using this method. A deceased Sim can also be revived as a zombie to prevent them from aging, although it is questionable if an immortal Sim would prefer such an existence.