It's a slippery, weasel word isn't it - sustainable? As you can see from Google's Ngram Viewer, It began sneaking into common parlance from about 1960, but didn't really take off dramatically until the mid-eighties. According to oxforddictionaries.com, this is what the word means.
- able to be maintained at a certain rate or level:sustainable fusion reactions
- conserving an ecological balance by avoiding depletion of natural resources:our fundamental commitment to sustainable development
- able to be upheld or defended:sustainable definitions of good educational practice
Yet in most cases, what we want to know is not whether something is sustainable, but whether it is actually sustained. If I cut down trees in a forest but don't plant more trees, then the wood comes from a sustainable source, but not a sustained source. My felling policy on the other hand is not sustainable, although it might be if regrowth occurs naturally and my rate of felling is no greater than the rate of natural regrowth.
All very picky I know, but why are we so casual about these things even where they supposedly matter?
Because sustainable is a political and marketing word I suppose - where sustainable ambiguity is the game.