A legacy application is a program or system which basically still works well and stable, but which is only maintained. Often the interface and the design of the application is outdated. Also, little new functionality is added.
Although there are a number of drawbacks to legacy software, it is possible that this software continues to be used due to costs. A vendor lock-in is also possible. Still, it often pays to replace legacy software, or by upgrading these systems to a recent version. This makes it possible to turn a legacy system into an active and up-to-date application again.