An edge case is a problem or situation that occurs only at an extreme (maximum or minimum) operating parameter.
For example, a stereo speaker might noticeably distort audio when played at maximum volume, even in the absence of any other extreme setting or condition.
For this reason, attempts to formalize good engineering standards often include information about edge cases.
Both methods aim at expanding the test coverage of a system, reducing the likelihood of unexpected edge cases.
In test-driven development, edge cases may be determined by system requirements and accounted for by tests, before writing code.