What is a relationship between two entities in an entity-relationship model?

In an entity-relationship model, a relationship between two entities refers to a connection that illustrates how the entities interact with one another within a system. This connection is crucial as it defines the business rules that govern the interactions and dependencies between the entities. For example, in a model representing a school, the relationship between "Students" and "Courses" could indicate that students can enroll in multiple courses, while each course can have multiple students.

This relationship could take various forms, such as one-to-one, one-to-many, or many-to-many, and is essential for accurately modeling the data requirements of the system being analyzed. Understanding these connections helps analysts design databases that reflect the true nature of the organizational processes, ensuring that data is structured correctly and can be queried effectively.

The other options do not correctly reflect the nature of relationships in an entity-relationship model. A data field represents a specific piece of data contained within an entity, rather than describing how entities are related. An external factor does not specifically pertain to the direct interactions between entities, and a contractual obligation implies a legal agreement, which does not specifically describe the nature of a relationship in the context of data modeling.