Define third normal form database?

The prohibition against repeating groups, on the other hand, is implicit in the way a relational database represents information. The nature of a database table in an RDBMS is that within each record of the table, a single field can only ever contain a single value. More precisely, it can only ever contain a single instance of the data type to which the column in question belongs. For example, if a column called Favorite Number is defined as an integer, each record is restricted to having one integer (or Null) in the Favourite Number field. Therefore – so long as primary keys are employed – it is technically impossible to design a relational database table that violates 1NF. Chris Date has argued that this holds true even when the RDBMS permits complex data types such as arrays.