# Glossary

The terms used in the Versia specification.

Terms used across this specification.

- **[Collection](/structures/collection)**: A paginated group of entities, such as a user's subscribers.
- **[Delegate](/extensions/delegation)**: The user that performs actions shown under another user's name.
- **[Delegator](/extensions/delegation)**: The user that a delegate's actions are shown as.
- **[Domain](/entities#domain)**: The hostname of an instance, used to identify it in the network.
- **[Domain handle](/extensions/domain-handles)**: A domain that a user is shown by, such as `@joe.example`, proven by a DNS record or a file on the domain that names the user.
- **[Entity](/entities#definition)**: A JSON object with a `type` and a `created_at`, such as a [User](/entities/user) or a [Subscribe](/entities/subscribe).
- **[Extension](/extensions)**: An optional, namespaced addition to the core protocol.
- **[Federation](/federation)**: The process of exchanging data between two or more instances.
- **Implementation**: A software application that implements the Versia Protocol. Mastodon is an implementation of ActivityPub.
- **[Inbox](/federation#inboxes)**: The endpoint that receives messages for all of an instance's users.
- **Instance**: An application deploying an implementation. `mastodon.social` is an instance of Mastodon. It is described by its [InstanceMetadata](/entities/instance-metadata).
- **[Interaction type](/extensions/interaction-controls#interaction-types)**: A kind of interaction with a Note that can be controlled, such as `pub.versia:social/reply`.
- **[Media](/structures/media)**: A file, such as an image, with its media type and metadata.
- **Receiver**: The instance a message is sent to.
- **[Reference](/entities#reference)**: A string of the form `domain:id`, which points to an entity.
- **[Reference Collection](/structures/collection#reference-collection)**: A Collection of references, used to list entities from other instances.
- **[RichText](/structures/rich-text)**: Formatted text, as one HTML string with a fixed set of elements.
- **[Requirement](/extensions#requirements)**: An extension that another extension builds on. An instance that lists an extension also lists the extensions it requires.
- **Sender**: The instance that sends a message.
- **[Structure](/entities#definition)**: A JSON object that does not meet the definition of an entity, such as a Media.
- **[Subscriber](/entities/subscribe)**: A user subscribed to an entity, such as another user.
- **[Subscription](/entities/subscribe)**: The link between a subscriber and the entity they are subscribed to, started with a Subscribe and ended with an Unsubscribe.
- **[Transient Entity](/entities#transient-entities)**: An entity with no `id`, which cannot be fetched or referenced.
- **[User address](/entities/user#addresses)**: A shorthand for a user, in the form `@username@instance`.