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$n_e$ Is the \textbf{fraction of Earth-like planets}.   A measure of how common are planets that can likely support biological life as we know it.  The definition is a little lose, but the idea is having a rocky planet in the so called Goldilocks or habitable zone, a range of distances from the host star where temperature at the surface is just about right for liquid water to be present. This might well be a restrictive definition of habitability, as life forms might thrive in very different environments from what we are used to. But one has to start somewhere, and liquid water seems to have played a decisive role as a catalyst for biological life on Earth.   The news came up just in the last few months: Earth-like planets are very common. Statistically speaking 1 in 5 planets around Sun-like stars could potentially support life \cite{Petigura_Howard_Marcy_2013}. And Sun-like stars are extremely common, resulting in about 10 billion of habitable planets just in the Galaxy. Galaxy and the closest to us potentially within 12 light years.  $n_e \approx$ 0.2