Fifth normal form, also known as 5NF or Project-Join Normal Form (PJNF), refers to a level of database normalization that aims to eliminate redundancy and data anomalies. There are various synonyms for the term fifth normal form as well, including "ultimate normal form", "domain-key normal form", "DKNF", and "compound key normal form". These terms all describe the same level of data normalization, which goes beyond the more commonly used levels of normalization such as first normal form (1NF) and third normal form (3NF). Achieving 5NF requires a deep understanding of the data and the relationships between different variables, and is generally recommended only for large, complex datasets that require a high level of data integrity.