An input value or output value which is on the boundary between equivalence classes, or an incremental distance either side of the boundary.
BS 7925-1.British Computer Society Specialist Interest Group in Software Testing (BCS SIGIST).

An important element in boundary value analysis. The boundary value is that value separating equivalence classes. An equivalence class is a set of data for which the softwares behaviour is assumed to be the same.

For example the spec states that a field should accept from £0.01 up to and including £10. A message is generated indicating whether the entered field is too low or too high.

The boundary values in this case are £0.01, £0.02, 9.99 £10.00 any amount in between is in the same equivalence class as they are all acceptable. Thus £2.48 and 9.98 should behave exactly the same.

£10.98 and £20 are part of another equivalence class as they are too high. A negative amount of -£20 forms part of another class as it generates another message.