One or more table fields (columns) that refer to the primary key field or fields in another table. A foreign key indicates how the tables are related — the data in the foreign key and primary key fields must match, though the field names do not have to be the same. For example, a Products table might contain the foreign key SupplierNumber, which relates to the primary key SupplierID in a Suppliers table.
A foreign key can be Null; if a foreign key consists of more than one field and any of those fields is Null, all the fields must be Null.