What term is used for older systems that a new system must interface with?

Legacy systems refer to older software or hardware that continues to be in use, often because it fulfills critical functions or because it is deeply integrated into an organization's operations. These systems can pose challenges when implementing new technology because the new system must be able to communicate and interact with the legacy systems.

Legacy systems can arise from previous technologies that are still operational but might not be efficient by modern standards. They may lack the scalability or flexibility of newer systems but often retain a massive amount of valuable data and functionality that must be maintained or transitioned in any new implementation. Understanding legacy systems is crucial for a system analyst, as they need to develop strategies for integration and data migration in order to ensure continuity in business functions.

In contrast, the other terms do not accurately capture the unique characteristics and context of systems that are older and still actively used within an organization.