Reclaimer

A "Reclaimer" is an individual who, whether by chance or by precedent, has been placed in charge of retrieving an item known as the Index, returning it to, and subsequently placing it into a control panel within a control center on a Halo structure to activate a massive superweapon. The word is first used as a title bestowed upon a Staff Sergeant Marvin Mobuto in Halo: The Flood and Master Chief in Halo: Combat Evolved by an artificial intelligence construct, designated 343 Guilty Spark by its creators, the Forerunners.

History
The only known role 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 sentient life within twenty-five thousand light years, and would also activate other Halos, causing the entire galaxy to be purged of sentient life. This would be done to prevent the spread of The Flood, which can only be accomplished by destroying all sentient life with a biomass sufficient enough to sustain them. This would then destroy the Flood via starvation.

343 Guilty Spark assumed, for unknown reasons, that the human soldier Master Chief was a Reclaimer. However, Commander Keyes and Sergeant Johnson were also called Reclaimers by 343 Guilty Spark. Penitent Tangent also refers to Master Chief as a Reclaimer, implying that all Monitors recognize him as such. In addition, in Halo: The Flood the Master Chief encounters the corpse of Sergeant Marvin Mobuto in the Library, whom Spark also refers to as a Reclaimer.

Other Interpretations and Speculation
''The above is one possible interpretation. It is not stated explicitly in either gameplay or literature that Reclaimer is even a title. Neither the Master Chief nor Commander Keyes are ever referred to as the Reclaimer. The Monitor always uses the terms Reclaimer, a Reclaimer, or this Reclaimer.''

Reclaimers are literally Reclaimers
Although only briefly referred to in Ghosts Of Onyx, one of the Sentinels speaks and refers to one of the Spartan-III's as a Reclaimer. This might mean that all Forerunners involved in the Halo project were Reclaimers, or the people that hid in the shield were meant to come back and "reclaim" all of the Forerunners technology after the Halo system fired.

Also, after the Spartan-III responds, the Sentinel reclassifies him as an "aboriginal sub-species." One could argue that the Spartan-III's are a lesser form, or sub-species, of the Spartan-IIs. However the more likely thing that happened was that the Spartan-III simply did not give the Sentinel the correct response. It is also important to note that SPARTAN-III's have extra genetic modifications that SPARTAN-II's did not receive. Their brains are significantly different than that of normal humans or even SPARTAN-II's. Their extra modified physiology and brains could be just different enough to classify them as something different than a human, i.e. a sub-species of human.

It was stated in Episode 5 of Iris that humans are meant to Reclaim the Ark and any other Forerunner technology.

Forerunners are human?
An interpretation of Reclaimer has been that humans were in fact Forerunners due to the comments said by 343 Guilty Spark and also due to similarities with the Halo installations and Earth and with the Ark being on Earth. But with the recent release of Iris it was confirmed when Episode 5 opened that humans were not Forerunners. This was also confirmed in the online comic Cradle of Life showing N'Chala watching the Forerunners build the Ark.