There seem to be two main uses of the term sustainability: environmental and financial. Since I am a touch of an idealist I think of sustainability in a broad scope. For something to be sustainable it has to be sustainable forever (or at least for the foreseeable future).
Environmental sustainability should be reserved for practises that stop future destruction of the environment, including biodiversity, even if every person on earth follows that practise. According to this definition, the use of oil for propulsion is not sustainable, nor is the consumption of animals for food. In short, the lifestyle of the average person in the “developed” world is not sustainable. This lifestyle has become the benchmark with which other nations measure their success. How can we expect them to lower their impact on the environment when we have had the benefit of 100+ years of unsustainable practises? Why should the emerging world not experience the kind of golden age that the first world has? Perhaps our golden age is coming to an end.