Legacy System (glossary)
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 (1). 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. (2)
(1) Arbutus software http://www.arbutussoftware.com/data-source/legacy-systems/. (2) wikipedia. http://en.wikipedia.org/wiki/Wiki
Seacord, Plakosh, Lewis. 2003. Modernizing Legacy Systems discusses the difficulties of updating or modifying a legacy system's software
Discussion as to why this is the "consensus" definition for the SEBoK.