/se-kənd si-stəm i-fekt/
n. (sometimes, more euphoniously, second-system syndrome) When one is designing the successor to a relatively small, elegant, and successful system, there is a tendency to become grandiose in one's success and design an elephantine feature-laden monstrosity. The term was first used by Fred Brooks in his classic The Mythical Man-Month: Essays on Software Engineering (Addison-Wesley, 1975; ISBN 0-201-00650-2). It described the jump from a set of nice, simple operating systems on the IBM 70xx series to OS/360 on the 360 series. A similar effect can also happen in an evolving system; see Brooks' Law, creeping elegance, creeping featurism.
See also Multics, OS/2, X, software bloat.
This version of the jargon lexicon has been described (with altogether too much truth for comfort) as an example of second-system effect run amok on jargon-1...