From Statistics Explained
Migration refers to the number of migrants, people changing their residence to or from a given area (usually a country) during a given time period (usually one year).
Immigrants are people arriving or returning from abroad to take up residence in a country for a certain period, having previously been resident elsewhere. According to the 1998 United Nations recommendations on the statistics of international migration (Revision 1), an individual is a long-term immigrant if he/she stays in his/her country of destination for a period of 12 months or more, having previously been resident elsewhere for 12 months or more.
Immigration is the number of immigrants for a given area during the year.
Emigrants are people leaving the country where they usually reside and effectively taking up residence in another country. According to the 1998 UN recommendations on the statistics of international migration (Revision 1), an individual is a long-term emigrant if he/she leaves his/her country of previous usual residence for a period of 12 months or more.
Emigration is the number of emigrants for a given area during the year.
Net migration is the difference between immigration to and emigration from a given area during the year (net migration is positive when there are more immigrants than emigrants and negative when there are more emigrants than immigrants). Since many countries either do not have accurate figures on immigration and emigration, or have no figures at all, net migration has to be estimated. It is usually estimated as the difference between the total population change and the natural increase during the year. Net migration gives no indication of the relative scale of the separate immigration and emigration flows to and from a country; a country may report low net migration but experience high immigration and emigration flows.
Crude rate of net migration is the ratio of net migration during the year to the average population in that year. The value is expressed per 1 000 inhabitants.