From HomoExcelsior

To change the properties of a planet to make it more earthlike, making it possible for humans or other terrestrial organisms to live unaided on it, for example by changing atmospheric composition, pressure, temperature or the climate and introducing a self-sustaining ecosystem. This will most probably be a very long-term project, probably requiring self-replicating technology and megascale engineering. So far Venus and especially Mars looks as the most promising candidates for terraforming in the solar system.

---