“Defect” is meant as a somewhat broader concept than “bug.” It includes, for example, problems like security vulnerabilities, pieces of code that won’t scale well if the number of transactions increases, and features that aren’t usable by people with disabilities. In today’s software development practices, developers write automated test scripts to test their code, and they run those tests constantly as they are developing the software. As a result, the automated tests are constantly finding defects and the developer is constantly adjusting the code. In other words, finding defects is a good and expected part of the process. The only bad defects are the ones that don’t get detected—the ones that make it through testing and remain in the code that is released to users, a.k.a. escaped defects.
— War and Peace and IT: Glossary, Mark Schwartz