Recent years have seen a flurry of work on sustainable computing and sustainable HCI, but it is unclear whether this body of work adheres to a meaningful definition of sustainability. In this paper, we review four interlocking frameworks that together provide a rigorous foundation for what constitutes sustainability. Each consecutive framework both builds upon and can loosely be seen as a refinement of the previous framework. More specifically, we leverage prominent ecological thinking from outside of computer science to inform what sustainability means in the context of computing. To this end, we re-evaluate some recent results from the field of sustainable HCI and offer thoughts on further research in the field.