A test case design technique for a component in which test cases are designed which include representatives of boundary values.
BS 7925-1.British Computer Society Specialist Interest Group in Software Testing (BCS SIGIST)

The tester needs to understand what will happen at the boundaries of the behaviour for each component. For example a field is required to accept amounts of money between £0 and £10. Does this mean up to and including £10 or £9.99? At the other end can an amount of £0 be acceptable? Is the boundary set at the pound or penny level?

The analysts decides the boundary values are £0, £0.01, £9.99 and £10. Once this has been established the tester can then set his test data.

The following tests are then to be executed.

A negative amount = rejectedl
0 = accepted (this is on the boundary)
null - rejected
£0.01 = accepted
£9.99 = accepted.
£10 rejected.

Items on the same side of the boundary value are said to be in the same equivalence class.

Thus at the top of the range we only have to test three values. £9.99, £10 both of which should be accepted and £10.01 which is rejected.

An important metric connected to this testing is Boundary Value Coverage