In an ERD, what does a double-lined rectangle represent?

In an Entity-Relationship Diagram (ERD), a double-lined rectangle represents a weak entity. A weak entity is an entity that cannot be uniquely identified by its attributes alone; it depends on another entity for identification through a foreign key. The double-lined rectangle distinguishes weak entities from strong entities, which are represented by single-lined rectangles. Understanding weak entities is crucial for modeling complex relationships and ensuring data integrity in database design.