A relation is a type of association that describes how two or more entities are related to each other. A relation can have different properties, such as cardinality, optionality, and directionality, that specify the nature and constraints of the association. A relation can also have attributes that store information about the association. For example, a relation between Employee and Project entities can have an attribute called Role that indicates the role of each employee in each project. A relation can be represented by a line connecting the entities in an entity-relationship diagram (ERD). References: Association and Associative entity - IBM, When to use Associative entities? - Stack Overflow, [Entity-Relationship Model - Tutorialspoint].