Second normal form (2NF) is a normal form used in database normalization. 2NF was originally defined by E.F. Codd in 1971.
To qualify for second normal form a relation must:
(1) be in first normal form (1NF)
(2) not have any non-prime attribute that is dependent on any proper subset of any candidate key of the relation. 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.