Maverick

Maverick (イレギュラー) is a term gived to Reploids and Mechaniloids that are a threat to humans, which can be caused by errors, viruses, reprogramming, or even by their own free will upon deciding that humans are too weak to live. They are usually countered by application of superior firepower to a fatal degree, and the Maverick Hunters were formed to enact this.

Mega Man X series
Originally, the term Maverick is used for a Reploid that goes berserk, violent, and insane, normally due to being infected by the Sigma Virus or other type of virus. They were the only enemies that would be in the levels.

Mega Man X4 brought the most attention to the exisistance of non-infected Reploids when the Repliforce are branded Mavericks in their attempts to gain independence from humanity.

In Mega Man X8, it's revealed by Lumine that the Reploids can become Maverick at will, without the need of being forced by viruses or other means.

It is interesting to note that the events of Mega Man X4 seem to indicate that the first Mavericks were NOT caused by the Sigma/Zero/Wily Virus. According to the cutscene showing Zero's origin. Sigma was the commander of the Maverick Hunters when he encountered Zero and broke the crystal on his forehead, apparently exposing himself and the rest of the world to the virus. However, there would be no need for Maverick Hunters if the virus had not been released yet. There are only two possibilities. The first Mavericks were caused by something else, such as a mechanical malfunction, flaws in Dr. Cain's redesign, or a decision to rebel against humans. The other possibility is that though Zero and the majority of the virus were contained in Dr. Wily's capsule, some traces of the virus may have been somehow spread throughout the world prior to Zero's discovery.

Mega Man Zero series
In the Mega Man Zero series, the term Maverick is used for any Reploid who opposes Neo Arcadia. Ciel created a Resistance to protect innocent Reploids labelled as Mavericks (though the only reploid characters who have the original definition of Maverick applied to them are Elpizo and Copy X, since they tried to harm humans [Elpizo wanted to make a Reploid-only nation, and Copy X, aside from antagonizing Ciel, was also responsible for ordering the missile strike on the human residential place; Area-3079, which resulted in the deaths of several humans]).

In Mega Man Zero 4, Dr. Weil takes control of Neo Arcadia after Copy X's death, and any who opposes him, human or Reploid, is considered a Maverick. This ended with Dr. Weil's death. Another example of Maverick at will is Elpizo from Mega Man Zero 2, who wants to destroy Neo Arcadia and all human beings for the good of the Reploids.

Mega Man ZX series
In the Mega Man ZX series, there are still Maverick attacks.