Test Coverage
A measure of how much of an application's code, features, or requirements are exercised by the test suite, usually expressed as a percentage.
Test coverage can be measured in several ways: code coverage (lines, branches, or statements executed by tests), requirements coverage (percentage of requirements with associated tests), or feature coverage (percentage of user-facing features tested). Code coverage tools like Istanbul/nyc, coverage.py, and JaCoCo instrument the code to track which lines execute during test runs.
High coverage does not guarantee quality. A test suite can execute every line of code without meaningfully asserting correct behavior. Coverage is best used as a tool to find untested areas rather than as a quality target in isolation.
Why It Matters for QA Teams
Coverage metrics reveal blind spots in the test suite, showing QA teams exactly which parts of the website have no automated safety net and are most at risk during changes.
Example
A coverage report shows that the authentication module has 92% branch coverage, but the password reset flow has only 34%. The team prioritizes writing tests for password reset, which happens to be a frequently reported source of support tickets.