A legacy system is a piece of computing equipment or software that is no longer supported. The system continues to satisfy the requirements for which it was first intended, but growth is not permitted. A legacy system will only ever do the current tasks for the business.
An outdated version of an older technology that is no longer in regular use is referred to as a legacy platform, sometimes known as a legacy operating system. Many businesses that employ computers have legacy platforms and applications that are essential to their operations.
In IT, a legacy system is outdated technology that's still being used by a business. Older computer hardware, software applications, programming languages and processes can all be considered legacy systems. Legacy systems often work for years, albeit less efficiently and without the latest upgrades and updates.