The IT systems that an enterprise has in place because of actions it has taken in the past (development or purchase). Because businesses are constantly changing (and technology is changing as well), at any given moment this legacy estate is likely to be somewhat out of date. In other words, it is carrying functional debt and technical debt. The legacy estate often acts as a drag on the enterprise’s nimbleness and business agility.
— War and Peace and IT: Glossary, Mark Schwartz