Migration and different concepts related to the process of migration are often determined by the context of research and specificity. The ever-changing global and local socio-economic and political scenarios are also shaping up the newly evolving migratory patterns in accordance with the times and context. To engage with research and a deeper understanding of migration-related issues, a basic knowledge of general concepts and terms would prove to be helpful, especially keeping up with recent developments. This chapter therefore gives a comprehensive snapshot of different concepts and definitions in the theory of migration, briefly explaining what each of these entails.