Abstract Illiberalism is often associated with the concept of “authoritarianism,” but their relation can be underspecified, confused, contradictory, or overlapping. This is in no small part due to the tricky conceptualization of authoritarianism itself, which holds to surprisingly different definitions across several social-scientific disciplines and deals with the same common problems of usage imprecision. This chapter conceptualizes the relationship between illiberalism and the several understandings of authoritarianism current in the mainstream academic literature. In doing so, it shows how the concept of authoritarianism understood as a form of political regime is the most useful for most scholars working on the subject of illiberalism, although in some ways also the most difficult to adhere. In support of this conclusion, the chapter reviews several prominent and influential alternative definitions of authoritarianism, including psychological-dispositional, psychological-behavioral, policy-ideological, and practice-process conceptualizations. It notes that these other variants of authoritarianism suffer from diverse, internal problems with conceptual coherency, parsimony, bias, rigor, and empirical replicability. Furthermore, they are particularly susceptible to obscuring or even hindering the empirical and theoretical application of illiberalism in scholarly study, although important exceptions and further avenues of exploration are noted as well. Familiarity with definitional problems associated with non-regime conceptualizations of authoritarianism will ultimately facilitate a more precise and nuanced scholarly research approach on illiberalism.