The term sustainability is a widely used and widely abused term. For many people, sustainability simply refers to the idea that human consumption and behavioural habbits are unsustainable, particularly with regards to diminishing resources and high levels of GHG emissions. While this is part of it, it is certainly not the full story.
The classic definition of sustainability is from the World Commission on Environment and Development, stating that sustainability is something that can 'meet the needs of the present without compromising the ability of future generations to meet their needs'. In practice sustainability is the idea that we must be responsible in our behaviour now to prevent problems in the future. This is not only referring to environmental responsibility, but also refers to human rights, ethics and even economic sustainability.