In its most basic form sustainability is relatively straightforwardto define:

A sustainable system is one where the amount of output from the system does not exceed the amount of input. Put another way: for a practice to be sustainable it must not use up any resource faster than that resource can be replaced.

This is the base definition that will be used in the investigations of Sustainability Research. When applied to specific instances the definition will be made more specific.

In common use sustainability acquires a more amorphous definition and often has connotations of environmentalism. When adding more specificity to the general definition given above, the specific definition manifestation will depend upon system you are interested in investigating.

For additional investigation of the term sustainability have a gander at Defining Sustainability in the analyses.