Sustainable

A practice is sustainable if it can be continued indefinitely within a system.

Indefinitely? Without limit, except as posed by external events truly beyond the control of those performing the practice that change the system.

Is there a limit beyond which it is not possible to continue? Assuming planet earth as the system, the sun is expected to put an upper limit on any biosphere after several billion years. Other events beyond our control that make life impossible on this planet may or may not occur. For purposes of this discussion sustainable civilization is able to continue on this planet while the sun provides a suitable environment i.e., for billions of years.

Wikipedia's articles on sustain and sustainability discuss various concepts and understandings of the term.

More sustainable or less unsustainable?

A lot of efforts for which the term "sustainable" or "more sustainable" has been used, are actually concerned with stretching the remaining time a practice can be continued. That does not solve the underlying problem, only provides more time to implement different practices that can indeed be continued. As such the terms "more sustainable" or "less unsustainable" are misleading. "More sustainable" improvements so far postpone serious problems by no more than one generation (If someone can find examples that do better, please add). That is a far cry from the millions of generations that could be possible. Such efforts, such as conserving resources (e.g., a car that is more efficient and/or used less), are worthwhile because they provide more time, but they must not divert attention from the effort to come up with sustainable solutions. No matter how much effort is spent on finding "more sustainable" improvements, someone will eventually have to come up with a solution that can be continued. The damage and thus the burden placed on future generations will be greater the longer that takes.

Creating sustainability

Creating sustainability is an entirely different task from stretching the time to continue existing practices. Nature shows that it can be done. Every civilization should try. None has succeeded so far. We know more now. Admitting that "more sustainable" is not sustainable will help to intensify efforts to create sustainable solutions.