Final answer:
A secondary key is a field or fields in a database table that can have duplicate values, unlike a primary key which is unique. It is used for organizing and retrieving data.
Step-by-step explanation:
A secondary key is one field or a combination of fields for which more than one row may have the same combination of values. Unlike a primary key, which must be unique for each row in a database table, a secondary key does not have to be unique and can be used for data retrieval purposes. An example of a secondary key could be a person's last name in a database table of customers. While the customer ID would typically serve as the primary key, the last name (a secondary key) could be shared by multiple customers, thereby allowing for the possibility of duplicate values across different rows.