Reincarnated Ones (転生者 tenseisha, lit. "Reincarnation") or Reincarnators are those individuals that have departed from their bodies due to death and have been reborn in a new body. There are two types of reincarnators: those that died and were reborn into the same world, Inhabitants (世界人 sekaijin), and those that died from a different world and were reborn here, Otherworlders (異世界人 isekaijin).
Reincarnated Inhabitants often have strong individual wills and abilities to be successful in reincarnation. Depending on the circumstances, the reincarnated person may or may not have their original memories. Those who do, end up gain additional knowledge or power compared to their previous state.
In the case for monsters such as the True Dragons, they are considered dead only if their cores are destroyed, otherwise, as beings of mass-energy, they can easily regenerate as their magic returns. However, even if they die, they will eventually "respawn" or be reborn into the world again somewhere else, albeit, without their memories. This is because the "world" only cares about the existences of the "Dragons" as "natural" existences and not as "individuals."
List of Known Reincarnated Inhabitants
Reincarnated Otherworlders also need powerful wills or some form of remnant thought (something like dying wishes), whether that's a conscious or subconscious wish. Unlike Reincarnated Inhabitants, it's possible for people who have incredibly powerful wills but still not reincarnate into this world, meaning that the process is either random, or there is some compatibility issue. It might also be related to the location of death since it was implied that Satoru Mikami died very close to the location where Shizue Izawa had been summoned from decades earlier.
Regardless, those who have reincarnated from a different world, because of their souls having to resist the pressure of traversing into a different plane of existence or reality, the reborn individual will oftentimes acquire one or more Unique Skills and/or several other abilities and resistances.