Theory:

Migration has been defined and interpreted differently by different experts.
In general, migration is defined as the permanent or semi-permanent change of residence of an individual or group of people over a significant distance. So, the term migration refers to the movement of people from one place to another.
United Nations Organization's Definition:
Migration is a form of geographical mobility of a population between a geographical unit to another, generally involving a permanent change of residence.

One of the most important aspects of social science is “Human Migration”. Since the earliest times, humanity has been on the move. Human beings in their early stages went in search of a better place to live with abundant food, water and shelter. This search gave rise to migration.
 
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Since the early men were unaware of agriculture, when food becomes scarce in a particular place, they shifted to another place. In this way, "Human Migration" started and maintained a close relationship with humankind.

When humans started agriculture, they lived a settled life (leaving the nomadic lifestyle or forest-dwelling) with domesticated animals in the fertile regions like deltas. As a result, there was a considerable change in the mobility of human beings.

They started living in permanent settlements leaving the nomadic life. During this stage, people continued to move in search of fertile land for cultivation from one region to another. Afterwards, the nature of mobility frequently changed over a period of time.