Traditionally, the habitable zone (also called HZ, Goldilocks zone, ecosphere or liquid water zone) refers to the zone around a star where it is neither too hot nor too cold for liquid water to exist. In reality, howewer, it can be modified by greenhouse gases, the possibility of life in parts of the temperature gradient on tidally locked worlds, volcanically heated subsurface oceans, high albedo cloud covers, and the possibility of non-water based life. And of course artificially habitable environments must be created in non-naturally-habitable places for successful space colonization (and, to a lesser extent, space travel).