Defining migration (especially internal migration) is a controversial activity. At one end of the spectrum migration is defined as the movement of people over some distance (or at least from one \"migration-defining area\" to another) and from one \"usual place of residence\" to another. At the other end of the spectrum the definition of migration discards the requirements that migration must involve a change of residence and a move across some distance. In this article a compromise between these two positions is suggested. Migration is defined here as the crossing of the boundary of a predefined spatial unit by one or more persons involved in a change of residence. The implications of this definition for the use of recent South African migration data are discussed. It is concluded that the data from Census '96 are comprehensive and expected to be inherently sound, despite a definition problem caused by the absence of predetermined \"migration defining areas\". The expected impact of this problem is assessed, and a partial compensation is suggested on the basis of findings from a national sample survey.