Net migration is the difference between

Net migration measures the overall effect of migration on a country's population. It is calculated as the difference between the number of immigrants (people moving into the country) and the number of emigrants (people leaving the country) over a given period.

The formula is:

\[ \text{Net Migration} = \text{Number of Immigrants} - \text{Number of Emigrants} \]

If the result is positive, more people entered the country than left, and the country experienced net immigration. If negative, more people left than arrived, indicating net emigration.

The other options do not define net migration. Per capita income and population are measures of economic output and population size respectively. Internal and external migration are categories of migration based on whether movement occurs within or across national borders, but their difference does not define net migration. Population and census are general demographic concepts unrelated to the migration calculation.

When revising population topics, remember that net migration, along with the birth rate and death rate, determines the overall change in a country's population size.