A legacy computer system is an older one. It is one that you are stuck with because replacing it would be too problematic. It would be complex, costly, and it may interfere with specific programs, and whatever protection may be in place.
Whenever you purchase something on credit card, conduct banking transactions, or take flight, computer systems dating back several decades are an integral part of the process. Banks, air traffic control systems, and many civil or military functions depend on technology that most of us would consider obsolete.
For example, the Pentagon's strategic Automated command and control system is part of a fifty -three- year old computer network constructed around the IBM series minicomputer. It was launched in 1976. You would need around four disks to store one average MP3 music file.
There are so many things that have changed in this world right now but the military still uses floppy disks and old computers. They do this because their systems have already been wired to do this. If they would upgrade, they need to redo the whole system and this may take some time.
There are other things that they consider to be more important as compared to making some changes to their system. They also know how crucial their system is. If they make a mistake, there are some serious repercussions that may occur. They would rather stick with one system that is already tried and tested for now.
The United States Strategic Command and many other industries and certain companies are still using the same old computers that they used decades ago. Many use the IBM computers, but they may be different versions. Floppy disks have been replaced with flash drives. However, in the 1990s, computer users would save the documents and information on floppy disks which were about 3 inches by 3 inches in length.
The American nuclear command and other nuclear plants still use the computers that were created when the nuclear system was created. Changing the computers would most likely mean to redo the entire system because the computer system has been functioning by using that computer and those floppy disks. Changing it might alter the way it functions which could be dangerous.