In astronomy, what is the habitable (or Goldilocks) zone?

There is only one planet we know of so far that is teeming with life––Earth. And on our planet, water is a critical ingredient for life as we know it. While astronomers still don't know whether there's life on other planets, they narrow the search for potentially habitable worlds using a handful of criteria. Because our blueprint for life is Earth, astronomers look for planets with Earth-like characteristics, like liquid water. But a celestial object can only orbit so close (like Mercury) or so far (like Pluto) from its star before water on its surface boils away or freezes. The 'Goldilocks Zone,' or habitable zone, is the range of distance with the right temperatures for water to remain liquid. Discoveries in the Goldilocks Zone, like Earth-size planet Kepler-186f, are what scientists hope will lead us to water––and one day life.