habitable zone

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The region in a stellar system where the surface temperatures of planets can sustain liquid water and therefore hold the highest probability for the existence of life.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun astronomy The zone around a star where a planet could experience temperatures like those on Earth, allowing for the possible existence of liquid water and of life.