Domain Model

I would define a domain as a collection of entities. A domain model is a structural representation of entities and the relationship among them to describe a problem or solution.

Entity

An entity is anything in real life that has a unique identity to distinguish from one another. It comprises a set of attributes to describe its characteristics and operations to achieve one or more stated purposes.

Relationships

Common relationships between entities are containment, aggregation, inheritance, implementation, and use or invocation.