Reclaimer

Summary
The term "Reclaimer" has been used by 343 Guilty Spark when refering to Master Chief and other humans. The following is a collection of theories as to the meaning of the word. It should be noted that this is all highly speculative.

Theories and Speculation
Reclaimer is a title, possibly bestowed by the Forerunners. The task of a Reclaimer is to recover the "Index" and return it to the Halo control center. Doing this would activate Halo's ability to destroy all organic life within twenty-five thousand light years, and would also activate other Halos, causing the entire galaxy to be purged of organic life. This would be done to prevent the spread of The Flood, which can only be accomplished by destroying all organic life it can survive on (in essence, destroying the Flood by starving them).

343 Guilty Spark assumed, for unknown reasons, that the UNSC soldier John 117 was a Reclaimer.

According to the Halo novels, there is a possibility that Reclaimers are chosen by someone who is still alive and active somewhere.

The above is one possible interpretation. It is not stated explicitly in either gameplay or literature that Reclaimer is even a title. Another interpretation is that a Reclaimer is the Forerunner terminology for a human. Neither The Master Chief nor Commander Keyes are ever reffered to as the Reclaimer. "Reclaimer", "A Reclaimer", and "This Reclaimer" have been used to refer to both characters. From this it is possible to assume that the Forerunners had knowledge of humans and delegated to them the task of Reclaiming Indexes and activating the rings, among many, many other speculative duties. It is also speculated that the Forerunners were human, or at least, very closely related, perhaps being direct ancestors or future form of humanity. There is evidence to prove this, such as 343 Guilty Spark's statements in Halo: Combat Evolved, and the fact that the Covenant have not asked humanity to join them.

Even this may assume too much, however. The Forerunner's plan to ensure the survival of civilization even after their own destruction was dependant on some sub-intelligent organic life surviving the activation of the Halo weapons. It was assumed that, given enough time, some life forms somewhere in the universe would evolve into a sapient form, rediscover space flight technology, and find the artifacts left behind. 343 Guilty Spark and his kind were created to oversee the plan and to serve as time capsules for the Forerunners. Presumably, in the the "Master Plan", the sapient creatures who eventually would discover the monitors were referred to as "Reclaimers", the newly evolved replacement intelligence that would reclaim Forerunner technology and history.

The Forerunners would not need to know the reclaimers' place of origin, what they would look like, or anything else about them. The time frame is not known. If, as 343 Guilty Spark says, the rings are in place over one hundred millennia, then when they were constructed mankind would have no civilization as yet. This assumes that the years referred to are terrestrial years. Gas Giants like Threshold are generally farther from their parent stars, and thus have a longer orbital period. The planet Neptune has a period of 164.9 years, for instance. Assuming an even hundred for Threshold, the length of time that 343 Guilty Spark was on Halo could have been over ten million years. This means that humanity's ancestors would have been just learning to jab sticks at things.

The ecosystems on the rings themselves and the life support on the Forerunner starships do suggest that the Forerunners were protein-DNA life forms, like humans and presumably all other life in the Halo universe (and all known life in the real universe). It may have been hoped, by the designers of the Forerunner plan, that a different form of life would evolve to replace them. A form that was immune to biological agents like the Flood. Regardless, it seems that "Reclaimer" is a term that Forerunner AI programming applies to any new sapient species it encounters. This does not explain why the term is not applied to the Covenant lifeforms, though it may be a sign of their link back to the Forerunners.