Recently there has been some discussion concerning a particular type of enhancement, namely ‘moral enhancement’. However, there is no consensus on what precisely constitutes moral enhancement, and as a result the concept is used and defined in a wide variety of ways. In this article, we develop a clarificatory taxonomy of these definitions and we identify the criteria that are used to delineate the concept. We think that the current definitions can be distinguished from each other by the criteria used for determining whether an intervention is indeed moral enhancement. For example, some definitions are broad and include moral enhancement by any means, while other definitions focus only on moral enhancement by means of specific types of intervention. Moreover, for some definitions it suffices for an intervention to be aimed or intended to morally enhance a person, while other definitions only refer to ‘moral enhancement’ in relation to interventions that are actually effective. For all these differences in definitions we discuss some of their implications. This shows that definitions are significantly less descriptive and more normative than they are regularly portrayed to be. We therefore hope that the taxonomy developed in this paper and the comments on the implications for the normative debate of the variety of definitions will provide conceptual clarity in a complex and highly interesting debate.