What is a legacy system?

In computer terminology the Word legacy is usually used as a legacy system, which represents an old computer system or application, which is used by the users, only because they don't want to change or redesign it. Normally legacy systems are considered problematic by the software professionals since it may not match with the technologies of the new era.

A "legacy" system is one that is either outdated for most practical purposes, or one that includes older forms of technology. A "legacy-free" PC is one that does not include older ports that would be necessary to support much older equipment, such as parallel ports, ISA slots, serial ports, PS/2 ports, and sometimes even VGA. A legacy system is an old computer system that is no longer in use, but may still have important information. It could also be one that is completely redundant. New systems either have to replace it or even connect to it. A company might have an old payroll system that is replaced by a new one. The old one becomes a legacy system. Data structures that are out of date by modern standards, may still be in operation in some computer systems because the newer systems had to be based on an older system. Having a legacy system may make it difficult to implement a new system or delay it. Good system design can integrate the older system and later modify the new system so that it is fully up to date with modern standards, without having lost any data from the old system. The old system can be retained on backup so that if it is needed it is accessible. Ideally, the new system should be able to safely and completely replace it, so that the old system is never needed again for any reason.