The Free On-line Dictionary of Computing (30 December 2018):

test coverage

    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.

   (2001-05-20)