Terceira

 is a human colony founded at some point prior to 2557. While initially a UEG colony, the planet eventually became home to a massive rebel community, leading to violent countermeasures on the part of the UNSC.

History
Little is known about Terceira prior to the end of the Human-Covenant war, although it was likely established at some point prior to that. During the post-war resurgence of Insurrectionist activity, Terceira filled with rebels and rebel sympathizers. Eventually the entire planet was seized by the United Rebel Front and the UNSC lost all control. Spartan-IV operatives field-testing the Oceanic MJOLNIR variant were deployed to recover the planet. It is unknown what the outcome of this deployment was.

Trivia

 * The planet is named for one of the main islands of Earth's Azores. It is therefore possible that Terceira's original inhabitants were of Portuguese descent.
 * It can be assumed that the planet was very watery, as the Oceanic armor type was developed specifically for Spartan operatives on the planet, although this is speculation.
 * The planet's name is Portuguese for "third."