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, and just plain changing it to fulfill the required goals for the planet in question. The UNSC used a terraforming process to make their colonies fit for human habitation.
Trivia
- Onyx is the only planet known to not have needed terraforming.