Terraforming

Terraforming is the process in which a planet is changed or "formed" to suit the needs of those terraforming the planet. It normally involves inserting plants and/or flora, introducing new minerals or substances, or other changes to fulfill the required goals for the planet in question. It can take up to several centuries to fully terraform an Earth sized planet, or even slightly larger. The UNSC uses terraforming processes to convert potential colonies into environments suitable for human habitation. The was the first UNSC ship to carry terraforming gear out of the Sol system.

The Flood terraform their habitats through natural processes, converting the atmosphere and using local biomass to spread itself until it can form a Flood Hive. Beginning as small spores, these eventually grow into larger clumps of biomass, which convert the areas atmosphere to what is presumably optimal for Flood expansion, and have been known to infect large areas, ranging from areas of Installation 05, to the planetoid High Charity.

Trivia

 * Onyx is the only planet known to not have needed terraforming.