While economists have been contributing to the discussion of various aspects of sustainability for decades, it is just recently that the term "sustainability economics" was used explicitly in the ecological, environmental, and resource economics community. Yet, the contributions that use the term "sustainability economics" do not refer to any explicit definition of the term, and are not obviously joined by common or unifying characteristics, such as subject focus, methodology, or institutional background. The question thus arises: What is "sustainability economics"? In this essay, we make an attempt at systematically defining and delineating what "sustainability economics" could be in terms of its normative foundation, aims, subject matter, ontology, epistemology, and genuine research agenda.