# What are entities?

Copy as Markdown[View as Markdown](/docs/development/guides/domains/entities/introduction.md)

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**Added in** `eventcatalog@2.36.0`

In Domain-Driven Design, an entity is an object with a unique identity that stays the same over time, even if its data changes. It’s defined by this identity, not just its properties. For example, a `Customer` with a unique ID is an entity, even if their name or address changes.

In Domain-Driven Design, entities are the core building blocks of a domain—they represent key concepts or things within that domain that have a unique identity. The domain defines the business logic and rules, and entities bring that logic to life by modeling real-world objects or roles, like `Order`, `Customer`, or `Invoice`. Each entity lives within a bounded context of the domain, ensuring it behaves consistently according to that part of the business.

**In EventCatalog entities are optional.**, if you don't want to add entity documentation you can skip this resource.

### How do entities work in EventCatalog?[​](#how-do-entities-work-in-eventcatalog "Direct link to How do entities work in EventCatalog?")

Entities are a "**optional**" resource in EventCatalog and like every other resource in EventCatalog they are defined in markdown files.

Entities can be added to your `domains` and `services`.