Big ball of mud
A big ball of mud is a software system that lacks a perceivable architecture. Although undesirable from a software engineering point of view, such systems are common in practice due to business pressures, developer turnover and code entropy. They are a type of design anti-pattern. In computer programs. The term was popularized in Brian Foote and Joseph Yoder's 1997 paper of the same name, which defines the term: Foote and Yoder have credited Brian Marick as the originator of the 'big ball of mud' term for this sort of architecture. Programmers in control of  a big ball of mud project are strongly encouraged to study it and to understand what it accomplishes, and to use this as a loose basis for a formal set of requirements for a well-designed system that could replace it. Technology shifts, such as client–server to web-based or file-based to database-based, may provide good reasons to start over from scratch. In relation to Lisp. In discussion of the Lisp programming language the term "big ball of mud" is used differently, in this case to describe the malleability of a Lisp system. In Lisp, it is  generally possible to: As a result of the confluence of these features, Lisp is extraordinarily flexible, even to such an extent that the language implementation itself may be completely rewritten during runtime (i.e. reflective metaprogramming), which can result in Lisp systems becoming "muddy" over time due to the fluidity and ease with which they may be extended and evolve through simple use. Metalinguistic abstraction, a prominent characteristic of Lisp, also allows programmers to develop entirely new and idiosyncratic conceptual vocabularies to describe the processes and functions their programs go through in tackling a problem domain, and can, if combined with  poor software documentation, result in Lisp systems that work quite well and are indeed quite well-structured from a design perspective, but are only comprehensible to the original coders, or to anyone else willing to invest the time to sift through layers of highly recursive code. Joel Moses has been credited with coining the phrase in the 1970s: APL is like a beautiful diamond – flawless, beautifully symmetrical. But you can't add anything to it. If you try to glue on another diamond, you don't get a bigger diamond. Lisp is like a ball of mud. Add more and  it's still a ball of mud – it still looks like Lisp. Moses strongly denies this, claiming he instead called Lisp a bean bag because it always returns to its original shape.