The ability to sustain
Personally I view the word 'sustainability' as a very dynamic one. There seems to be no way of explaining it in it's entirety, rather it's a feeling of living life in low impact manner.
It can loosely be defined with diagrams and graphs but ultimately it's a personal thing.
There are billions of people on this planet and each of them has there own set of desires and needs, some a lot more than others.
To create a sustainable world it would need to cater for every individual and adapt to their needs. It should be a flexible system, allowing people as much freedom as possible and encouraging responsible use of resources, money etc.
Striving towards a sustainable world is as much a physical goal as it is cultural, linked directly to our mentality and outlook in everyday life.