If you break down the word, it can be simply defined as something that is 'able' to be 'sustained'. To me that looks like an enclosed system that neither takes nor produces more than necessary to be cohesive with nature and physics. We do not live in a world that is accommodating to this kind of mindset, but instead a consumer society where we are treated one step above, or below, animals; depending on how you look at it. We are so far removed from our humanity that even a "natural habitat" is almost asking too much. I believe a farm is a natural habitat for humans because the lifestyle involves fresh air, plants and animals, physical activity and mental stimulation daily. Now, owning your own land has become less of a priority as renting has become more realistic for most wages. People are too "busy" to get fresh air or exercise every day, and some children only ever see cows on tv and milk cartons, much less have an understanding or compassion for local wildlife.
Some ways we can take steps toward sustainability in our own lives can be as small as growing your own fruits and vegetables, to building and using a compost bin for your leftover biodegradable waste. Grow some native plants and feeder plants for local wildlife. Reduce the amount of waste you and our household produces and change any wasteful habits; like overspending or not reusing items with value. Of course there are so many ways to go green and start living more consciously, but the core of action should always be changes that are sustainable.
"Be the change you wish to see in the world." -Gandhi