This briefing considers the different ways in which one can understand who counts as a migrant in the UK and the implications of using different definitions. Definitions of ‘migrant’ vary among different data sources, and between datasets and law. Among other possibilities, migrants may be defined as foreign-born, foreign nationals, or people who have moved to the UK for a year or more. Different definitions have significant consequences for data, both in terms of numbers of migrants (stocks and flows), and for the analysis of the impacts of migration. The use of the term in public debate is extremely loose and often conflates issues of immigration, race/ethnicity, and asylum. Conflicting definitions pose challenges for policy, particularly since many ‘migrants’ are not subject to immigration control and legislation. When defined by intentions of longer-term length of stay, more migrations have been leaving the UK than arriving in the UK.