Legacy System (glossary)
(1) A system that has been in operation for a long time, and whose functions are too essential to be disrupted by upgrading or integration with another system. (Arbutus Software 2012)
(2) A legacy system is an old method, technology, computer system, or application program that continues to be used, typically because it still functions for the users' needs, even though newer technology or more efficient methods of performing a task are now available. A legacy system may include procedures or terminology which are no longer relevant in the current context, and may hinder or confuse understanding of the methods or technologies used. (Wikipedia Contributors 2012)
(1) Arbutus Software. "Legacy Systems." Accessed September 12, 2012. Available at: http://www.arbutussoftware.com/data-source/legacy-systems/.
(2) Wikipedia contributors. "Legacy system." Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Legacy_system&oldid=511592171 (accessed September 13, 2012).
Seacord, R.C., D. Plakosh, and G.A. Lewis. 2003. Modernizing Legacy Systems: Software Technologies, Engineering Processes, and Business Practices. Boston, MA, USA: Pearson Education.
discusses the difficulties of updating or modifying a legacy system's software SEBoK v. 1.9.1 released 5 October 2018