In software development, or any other IT field (e.g., Infrastructure, Networking, etc.) technical debt (also known as design debt or code debt) is the implied cost of future reworking required when choosing an easy but limited solution instead of a better approach that could take more time.
Analogous with monetary debt, if technical debt is not repaid, it can accumulate "interest", making it harder to implement changes. Unaddressed technical debt increases software entropy and cost of further rework. Similarly to monetary debt, technical debt is not necessarily a bad thing, and sometimes (e.g. as a proof-of-concept) is required to move projects forward. On the other hand, some experts claim that the "technical debt" metaphor tends to minimize the ramifications, which results in insufficient prioritization of the necessary work to correct it.
As a change is started on a codebase, there is often the need to make other coordinated changes in other parts of the codebase or documentation. Changes required that are not completed are considered debt, and until paid, will incur interest on top of interest, making it cumbersome to build a project. Although the term is primarily used in software development, it can also be applied to other professions.
In a Dagstuhl seminar held in 2016, technical debt was defined by academic and industrial experts of the topic as follows: "In software-intensive systems, technical debt is a collection of design or implementation constructs that are expedient in the short term, but set up a technical context that can make future changes more costly or impossible. Technical debt presents an actual or contingent liability whose impact is limited to internal system qualities, primarily maintainability and evolvability."
Causes
Common causes of technical debt include: Ongoing development, long series of project enhancements over time renders old solutions sub-optimal. Insufficient up-front definition, where requirements are still being defined during development, development starts before any design takes place. This is done to save time but often has to be reworked later. Business pressures, where the business considers getting something released sooner before the necessary changes are complete, builds up technical debt involving those uncompleted changes. Lack of process or understanding, where businesses are blind to the concept of technical debt, and make decisions without considering the implications. Tightly coupled components, where functions are not modular, the software is not flexible enough to adapt to changes in business needs. Lack of a test suite, which encourages quick and risky band-aid bug fixes. Lack of software documentation, where code is created without supporting documentation. The work to create documentation represents debt. Lack of collaboration, where knowledge isn't shared around the organization and business efficiency suffers, or junior developers are not properly mentored. Parallel development on multiple branches accrues technical debt because of the work required to merge the changes into a single source base. The more changes done in isolation, the more debt. Deferred refactoring; As the requirements for a project evolve, it may become clear that parts of the code have become inefficient or difficult to edit and must be refactored in order to support future requirements. The longer refactoring is delayed, and the more code is added, the bigger the debt. Lack of alignment to standards, where industry standard features, frameworks, and technologies are ignored. Eventually integration with standards will come and doing so sooner will cost less (similar to "delayed refactoring"). Lack of knowledge, when the developer doesn't know how to write elegant code. Lack of ownership, when outsourced software efforts result in in-house engineering being required to refactor or rewrite outsourced code. Poor technological leadership, where poorly thought out commands are handed down the chain of command. Last minute specification changes. These have potential to percolate throughout a project, but there is insufficient time or budget to document and test the changes.