Code coverage (or 'test coverage', or just 'coverage') is a measurement, usually expressed as a percentage, of how much of your source-code is being executed by your test suite.

In general, a code coverage system collects information about the running program, then combines that with source information to generate a report on the test suite's code coverage.

This information can then be used to improve the quality of the test suite, either by adding tests or modifying existing tests to increase coverage.