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. The UNSC use 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 solar 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.