Boyce-Codd Normal Form (BCNF)

Third normal form removes most redundancy, and for most tables it is enough.

Then a table arrives where every attribute is part of some candidate key. Third normal form looks at it, finds nothing to complain about, and passes it. The table still stores one fact twice.

Boyce-Codd normal form is the stricter rule that catches those tables. It is sometimes called 3.5NF, because it sits between 3NF and 4NF in strength.

The rule

A table is in BCNF when, for every non-trivial functional dependency X -> Y that holds on it, X is a superkey.

That is the whole definition