The percentage of boundary values of the component's equivalence classes which have been exercised by a test case suite.
BS 7925-1.British Computer Society Specialist Interest Group in Software Testing (BCS SIGIST).

A metric indicating how much testing is required. It is important because a lot of defects are discovered at the boundary values. Thus if none or very few boundary value tests are left, then the risk of failure has been significantly reduced.

See also boundary value analysis.

A related metric is Equivalent Partition Coverage