Terraforming was a form of planetary engineering in which a planet or moon was modified through artificial means to match the atmosphere, temperature and ecology of a Class M world to make it more habitable for humanoid life.
Orions terraformed their own homeworld of Botchok (Rigel VIII) following the Atom War and the Long Winter, in a project known as the First Stage. It was not a quality effort, and records of the project were fragmented. The rebuilt ecology of Botchok was not entirely successful, as the planet's terraforming required continual re-adjustment into modern times, but the planet almost entirely recovered. Orions subsequently gained a reputation for terraforming; the Pergue imported them to Syrenya to repeat their successes. Syrenya was the earliest completely documented example of terraforming known to Federation scholars.
There was a faction within the Federation which argued that terraforming was a violation of the Prime Directive, as it eliminated the potential for the development of life and a subsequent civilization. The Worlds Within Worlds project was thus developed to offer an alternative to terraforming. (NF novel: Missing in Action)