This article examines the classification of migration and the political implications of its vocabulary. It begins with an analysis of the challenge that the topic of migration poses to traditional methods of classification. These challenges are evident in the case study of the 2016 petition to change the subject heading of “Illegal aliens” at the Library of Congress, the Library's proposal to replace the term with “noncitizens” and “illegal immigration”, and subsequent pressure from the House of Representatives to reinstate the original phrasing in order to match the terminology of federal laws.