Legacy Systems refers to out-dated computer systems, programming languages or software. Legacy implies that the system is out of date or in need of replacement, however, it may be delivering the expected results, so the business or individual owner does not want to upgrade or update.
For the banks it often means, a system that cannot be taken out of service, and the cost of designing a new system with a similar availability level is very high, for example transaction monitoring tools built in the .net framework.