Sustainability, Quasi-Sustainability, and Feasibility
An energy technology is sustainable if and only if ERoEI* (E-R-O-E-I star) is no less than 1.0. An entire society is sustainable if and only if the compound ERoEI* of its entire mix of energy technologies is no less than 1.0. Early on, recognizing that a community can persist for quite a long time if most of the characteristics of ERoEI* are satisfied, we considered quasi-sustainability; that is, during a transition period between fossil fuels and renewable energy, we must tolerate some slight environmental destruction and diminution of our storehouses of essential natural resources because of the large proportion of the energy investment for most renewable energy technologies that must be paid before any energy is returned.
I think it might be useful to define "feasibility" as something different from sustainability. For example, we might say that a renewable energy is feasible if no more characteristics of ERoEI* are relaxed than are consistent with the community standards and laws of the land currently.