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. I 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.

Reclaimer is another word for human
Another interpretation is that a Reclaimer is the Forerunner terminology for a human. From this it is possible to assume that the Forerunners had knowledge of humans and delegated to them the task of reclaiming Indices and activating the rings, among many other speculative duties. This theory is supported by the scene when an Onyx Sentinel chasing Spartan-III G099 attempted to communicate with him, calling him a Reclaimer. Also, the Brute Tartarus ordered Miranda Keyes to insert the Index, instead of doing it himself, further supporting this theory.

The recent conclusion of the Iris ARG hinted that humans are reclaimers because the Forerunners discovered humanity soon before they activated the Halo array, choosing to protect the humans rather than let them die out too.

Forerunners are human
An interpratation 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 that the Forerunners discovered humans on the Earth and that they held the key to their own mysteries, so us humans are a missing link to the Forerunner. This was also comfirmed in the online comic Cradle of Life showing N'Chala watching the Forerunners build the Ark. In Halo 3 the Prophet of Truth and 343 Guilty Spark hint that humans are Forerunners, although Beungie have left exactly what they meant ambiguous.

Enhanced means Reclaimer
Another interpretation is that is not the species, but their cybernetic implants that mark the Master Chief and Commander Keyes as Reclaimers. Gravemind also referred to the Master Chief's "machine and nerve" mind as "concluded", which may mean it is closer to some arbitrary standard (the Forerunners?) than a merely organic mind. As all UNSC personnel have cybernetic implants of varying qualities, this may mean that a civilian human would not be regarded as a Reclaimer. This is, as is much of the Halo universe, all highly speculative.

(Given the situation, machine and nerve could simply be a reference to his cybernetic enhancements and has his mind 'concluded' in ending this war, while the Arbiter was 'merely flesh and faith and is the more deluded' in that he had no modifications and was blind in his faith towards the Covenant beliefs)

Time Travel
Working with the same quote, from the Two Betrayals (Levels). "Why do you hesitate to do what you've already done?" this line perhaps suggests that as a Reclaimer, the Chief would be referred to as a Forerunner so therefore has done it generally as a race.

This theory originated with the above quote, and because of Bungie's previous popular First-Person-Shooter series, Marathon. In the last game of the Marathon series, the protagonist (a MJOLNIR Mark IV Military Cyborg)uses an alien facility to travel back and forth, not just through time but between alternate dimensions, in order to stop an ancient evil from being unleashed. The seeming similarities between Halo and Marathon have led to a portion declaring that the Master Chief has activated Halo - by going back in time, later in the series.

Fortunately, Joseph Staten himself, one of Bungies lead writers, has said that time travel is "corny." In Halo 3, the last of the "Master Chief Trilogy," the Spartan makes absolutely no use of time travel at all. Moreover, only one of the novels even mentions time travel, Halo: First Strike, and then only as a means of getting the Master Chief back to Earth in the short period of time set by Bungie. Time travel is the least likely theory concerning the Reclaimers, and likely plays no part at all in the rest of the Halo series.

Insanity
However, the above can be contradicted with the fact that 343 Guilty Spark is arguably insane. When he made the quote "Why do you hesitate to do what you've already done?" and recited a previous conversation he had with an unnamed individual, Spark may very well have been referring to a different Reclaimer from when Halo was first activated and, due to his insanity, conceived the Master Chief as the original Reclaimer. Or it could be that the Chief already put the index in and tried to activate the ring. However Cortana took the index from inside the mainframe and did not allow Halo to be activated. So when 343 Guilty Spark says "Why do you hesitate to do what you've already done?" it is referring to the Chief already trying to activate Halo. This too, however can be countered by the fact that Guilty Spark then goes to say "Last time when you asked me, 'would I do it?' Having had considerable time to ponder your query, my answer has not changed. We must activate the rings." he does in fact believe he has encountered the Master Chief before, or someone who greatly resembles him.

A problem with this theory is that the monitor of Installation 05 also referred to MC, and not the Arbiter, as a Reclaimer. This occurs during the cut-scene where Gravemind is introduced. Instead, the Covenant, or possibly just the Elites, are referred to as "Meddlers," taking a quote from the final scene in The Oracle, where Spark says "Why do you meddlers insist on using such inaccurate verbiage?," to the Arbiter