Despite widespread concern over the alleged rise of conspiracy theories, scholars continue to disagree whether it is possible to distinguish specific kinds of conspiracist accounts that can justifiably be denounced as objectionable. In this article, we review scholarship from multiple disciplines to develop a composite definition of “conspiracy theories proper” (CTP) that violate fundamental norms of democratic discourse. Besides referring to grand conspiracies to account for social phenomena, we argue, such conspiracy theories: (a) assume conspirators’ pervasive control over events and information, (b) construct dissent as a Manichean binary, and (c) employ an elusive, dogmatic epistemology. We discuss the operational potential and limitations of our definition using news user talkbacks on the U.S., British and German online editions of Russia Today (RT), a popular platform among proponents of out-of-mainstream political views. Identifying key operational challenges in the classification of natural discourse, we sketch avenues toward a more rigorous study of contentious political talk.