How is a many-to-many relationship defined in database design?

A many-to-many relationship in database design occurs when multiple records in one table can be associated with multiple records in another table. This type of relationship is critical for representing complex interrelations between entities where the association is not simply one-to-one or one-to-many.

For instance, in a database representing students and courses, a single student can enroll in multiple courses, while each course can have multiple students enrolled. This interconnectivity necessitates the use of a junction table to facilitate the many-to-many relationship efficiently and to maintain data integrity. The junction table will typically contain foreign keys that reference the primary keys of the two tables involved, effectively allowing the recording of each unique combination of the two entities.

In contrast, the other descriptions do not capture the essence of a many-to-many relationship. One-to-many relationships describe scenarios where a single record in one table is associated with multiple records in another, while many-to-one relationships describe the reverse. Lastly, the notion of uniqueness pertains to the attributes of a table, which is unrelated to the relational dynamic represented by many-to-many relationships.