Entities (a.k.a. reference objects)

An object defined primarily by its identity is called an ENTITY. ENTITIES have special modeling and design considerations. They have life cycles that can radically change their form and content, but a thread of continuity must be maintained. Their identities must be defined so that they can be effectively tracked. Their class definitions, responsibilities, attributes, and associations should revolve around who they are, rather than the particular attributes they carry. Even for ENTITIES that don't transform so radically or have such complicated life cycles, placing them in the semantic category leads to more lucid models and more robust implementations.

The model must define what it means to be the same thing.

Eric Evans, "A Model Expressed in Software", in Domain-Driven Design: Tackling Complexity in the Heart of Software, 91-92.