Sustainable open source projects are those that are capable of supporting themselves. Simply put, they are able to meet their ongoing costs.
However, from the viewpoint of selection and procurement, sustainability also means that the project is capable of delivering improvements and fixing problems with its products in a timely manner, and that the project itself has a reasonable prospect of continuing into the future.
Elsewhere on our site you can find articles describing some of the many formal approaches to evaluating open source software as part of the Software Sustainability Maturity Model.
Read more at OpenSource.com