The term "population dividend" refers to a demographic phenomenon rather than the total number of population (option 1). It is the benefit a country can gain from having a youthful age structure of a population (option 2).

A youthful age structure means that a large proportion of the population is made up of young people who are in their productive age. This can result in positive economic and social outcomes for a country. The youthful population can contribute to economic growth by being a productive workforce, increasing innovation, and driving consumption. It can also support social development by having a larger pool of individuals to invest in education and healthcare.

Option 3, which states that the population dividend refers to a relatively high proportion of experienced aged people, is not correct. The population dividend is specifically related to the benefits of having a youthful population.

Option 4, migration from richer region to poorer region, is not related to the concept of population dividend. It is unrelated to the age structure of a population and the benefits that can come from it.

In summary, the correct answer is option 2, as it accurately describes the concept of population dividend.