A computer, software program, network, or other computer equipment that remains in use after a business or organization installs new systems. Compatibility with legacy systems is an important consideration when a new version is installed. For example, will a new spreadsheet software release be able to read the existing business records without expensive and time-consuming conversion to a new format? Legacy systems in many organizations are based on mainframe computers, which may be either augmented or slowly replaced by client/server architectures.