Returning to the picture I chose (the children cleaning the globe), my definition of sustainability is now thus:
Sustainability is an interdependent combination of concerns for the environment, the human condition, and for economic prosperity. The children in the picture are focused on maintaining the earth, not just for the environment, but for the personal conditions in their own futures (i.e. in health, etc.).
My previous definition only considered the environmental aspect of this definition, but now I recognize that it is a three-pronged system in place.