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 systematically define and delineate "sustainability economics" in terms of its normative foundation, aims, subject matter, ontology, and genuine research agenda.