Final answer:
A relation is in 3NF if it is in 2NF and there are no non-key attributes determined by another non-key attribute.
Step-by-step explanation:
A relation is in 3NF (Third Normal Form) if and only if it is in 2NF (Second Normal Form) and there are no non-key attributes determined by another non-key attribute. In simpler terms, a relation is in 3NF if there are no transitive functional dependencies involving non-key attributes.
For example, let's say we have a relation with attributes A, B, and C. If A determines B, and B determines C, but A does not determine C, then the relation is not in 3NF because there is a transitive functional dependency involving non-key attributes.
To summarize, a relation is in 3NF if it is in 2NF and there are no non-key attributes determined by another non-key attribute.