Data migration is the process of moving data from one location to another. This is typically done when moving off of one storage or processing system to another. A common example of data migration is the movement of data off of a legacy computing platform (like a mainframe) to a more modern platform (like a public cloud).
The data migration process often happens when a new system is introduced, or there is a new location for the data. It sometimes means that legacy systems are being replaced or augmented by new applications that will share the same dataset. It also happens when a company moves away from having onsite infrastructure and applications on-site to cloud-based applications and storage.