Second normal form (2NF) is a normal form used in database normalization. 2NF was originally defined by E. F. Codd in 1971.

A relation is in the 'second normal form if it fulfills the following two requirements:

1. It is in first normal form.
2. It does not have any non-prime attribute that is functionally dependent on any proper subset of any candidate key of the relation (i.e. it lacks partial dependencies). A non-prime attribute of a relation is an attribute that is not a part of any candidate key of the relation.

Put simply, a relation is in 2NF if it is in 1NF and every non-prime attribute of the relation is dependent on the whole of every candidate key. Note that it does not put any restriction on the non-prime to non-prime attribute dependency. That is addressed in third normal form.