A legacy system means an outdated computer software/application, equipment/hardware, or  technology still used in daily operations. The system still fulfils the requirements for which it was  initially designed for, but that system has no longer support and maintenance and is also restricted in  terms of development. However, they can not conveniently be replaced. For example, Windows XP,  Windows Vista & Windows 7 are legacy operating systems after Microsoft has stopped security &  other updates and assistance for it. More than 2 million devices are still running on this operating  system.

Legacy systems are becoming more and more outdated. They were implemented as a custom  solution long ago and may not have been designed with security in mind. This is why they are  vulnerable to cyber attacks, data breaches, and other security risks. It is important for organisations  need to take the necessary precautions to make sure that their legacy systems are updated on a  regular basis. They should also invest in cybersecurity practices because this will help them avoid  costly data breaches and other security incidents that can be detrimental to their business.

Click below link to read more Legacy System Definition  :
https://avendata.com/blog/legacy-systems-definition-and-why-do-we-need-to-archive-legacy-systems/