ABSTRACT
Migration means a change in the location of where people live. This simple statement, however, obscures a number of technical issues, the most important of which are the size of the spatial units used to define the boundaries over which a person must move and the time that a person needs to remain at a destination in order to be defined as a migrant. The United Nations defines an international migrant as someone who has moved across an international boundary for 12 months or more in coming to its estimate of a stock of international migration of 257.7 million in 2017 (United Nations 2019a). However, a move across an international boundary provides only a very general idea of the distance travelled: it could be many thousands of kilometres across an ocean or simply a short move across a land border between contiguous countries. The International Organization for Migration (IOM) adopts a more inclusive definition as those who have changed their habitual place of residence by moving across a border, regardless of length of stay (IOM 2018). Both definitions make the distinction between those moving across administrative boundaries within a country (internal migration) and those moving across international borders (international migration). Neither of these definitions, however, includes any reason for the move, whether ‘forced’ or ‘voluntary’, or any measure of the legality of the move. The estimates are simply numbers of all those who have moved from one spatial unit, however defined, to another.
