Full Idea
A 'supervaluation' says a proposition is true if it is true in all classical extensions of the original partial valuation. Thus 'A or not-A' has no valuation for an empty name, but if 'extended' to make A true or not-true, not-A always has opposite value.
Gist of Idea
A 'supervaluation' gives a proposition consistent truth-value for classical assignments
Source
Stephen Read (Thinking About Logic [1995], Ch.5)
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.140