Sustainability to me is keeping things at a manageable level, in all aspects of life. The main thing I think of is a sustainable environment, one where things have a positive impact on the immediate future.
Environmental sustainability is the rates of renewable resource harvest, pollution creation, and non-renewable resource depletion that can be continued indefinitely. If they cannot be continued indefinitely then they are not sustainable. There are three main pillars of sustainability are economic development, social development and environmental protection.