The concept of sustainability relates to the maintenance and enhancement of environmental, social and economic resources, in order to meet the needs of current and future generations. The three components of sustainability are: (1) Environmental sustainability - which requires that natural capital remains intact. This means that the source and sink functions of the environment should not be degraded. Therefore, the extraction of renewable resources should not exceed the rate at which they are renewed, and the absorptive capacity of the environment to assimilate wastes should not be exceeded. Furthermore, the extraction of non-renewable resources should be minimised and should not exceed agreed minimum strategic levels. (2) Social sustainability - which requires that the cohesion of society and its ability to work towards common goals be maintained. Individual needs, such as those for health and well-being, nutrition, shelter, education and cultural expression should be met. (3) Economic sustainability - which occurs when development, which moves towards social and environmental sustainability, is financially feasible.
Source: NCVER 2013, Australia

Sustainability is the capacity of the project to continue to exist and function beyond the end of the contract. The project results are used and exploited continuously. Sustainability of results means use and exploitation of results in the long term.
Source: EU Commission LLP 2007-2013, Europe

The ability of something (e.g. a policy, a system or an institution) to remain maintained at a certain standard (e.g. quality of education).
Source: Ministry of Labour 2005, Jordan