A measure of the proportion of a program exercised by a test suite, usually expressed as a percentage. This will typically involve collecting information about which parts of a program are actually executed when running the test suite in order to identify which branches of conditional statements which have been taken.
The most basic level of test coverage is code coverage testing and the most methodical is path coverage testing. Some intermediate levels of test coverage exist, but are rarely used.
The standard Unix tool for measuring test coverage is tcov, which annotates C or Fortran source with the results of a test coverage analysis. GCT is a GNU equivalent.