Population viability refers to the probability of survival of a population over a given period of time. Biologists and ecologists often discuss it in terms of minimum viable population, which is the smallest number of individuals needed to ensure the reach a high probability of survival (typically >95%) over long period of time, and use population viability analysis (PVA) to model population dynamics. Scientists can use such analysis as a tool to help restore stability to populations of endangered species. See further explanation of PVA here.
Sources:
http://www.eoearth.org/view/article/51cbedc57896bb431f693ccb/
http://www.eoearth.org/view/article/154633/