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Glossary

A comprehensive glossary of terms used in ActivityPub, ActivityStreams, and the Fediverse.

A​

Activity​

An action performed by an Actor. Activities are the core unit of communication in ActivityPub. Examples include Create, Follow, Like, Announce, and Delete.

See also: Activity Types, Activity Properties

ActivityPub​

A W3C Recommendation protocol for decentralized social networking. It defines how servers communicate (Server-to-Server) and how clients interact with servers (Client-to-Server).

See also: ActivityPub Overview

ActivityStreams 2.0​

A JSON-LD based vocabulary for representing social data. ActivityPub uses ActivityStreams as its data format.

See also: ActivityStreams Overview

Actor​

An entity that can perform activities. Actors have an inbox for receiving activities and an outbox for publishing them. Types include Person, Group, Service, Application, and Organization.

See also: Actor Types, Building an Actor

Addressing​

The mechanism for specifying recipients of an activity using to, cc, bto, bcc, and audience properties.

Announce​

An activity type used to share or "boost" content. Equivalent to retweet/reblog/boost on various platforms.

See also: Likes and Shares

Application​

An Actor type representing a software application, often used for bots or automated services.

B​

Backfill​

The process of fetching historical content when a new follow relationship is established.

Block​

An activity that prevents interaction between actors. Blocked actors cannot see content or interact with the blocking actor.

Boost​

Common term for sharing/reblogging content. Implemented as an Announce activity in ActivityPub.

BlurHash​

A compact representation of an image placeholder, used to show a blurred preview while the full image loads. Commonly used by Mastodon and Pixelfed.

C​

C2S (Client-to-Server)​

The ActivityPub protocol for client applications to interact with servers. Less commonly implemented than S2S.

See also: Client-to-Server

Collection​

An ordered or unordered set of objects. Used for followers lists, outboxes, and other aggregations.

See also: Collection Types

Content Negotiation​

The HTTP mechanism for requesting different representations of a resource. ActivityPub uses this to serve JSON-LD to federation requests and HTML to browsers.

Accept: application/activity+json

Content Warning (CW)​

A summary/spoiler text that hides the main content behind a click. Implemented using the summary property with sensitive: true.

Context​

In JSON-LD, the @context property defines the vocabulary used in a document. ActivityPub uses https://www.w3.org/ns/activitystreams as its primary context.

D​

Dereference​

The process of fetching a remote object by its URL/ID. When you receive an activity with an object ID, you dereference it to get the full object.

Digest​

An HTTP header containing a hash of the request body, used with HTTP Signatures to ensure message integrity.

Digest: SHA-256=X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=

Direct Message (DM)​

A private message addressed only to specific actors, with no public or followers visibility.

See also: Direct Messages

Dislike​

An activity expressing negative sentiment. Used by Lemmy for downvotes but not supported by Mastodon.

E​

Emoji​

Custom emoji supported via the Emoji type (Mastodon extension). Rendered inline in content using :shortcode: syntax.

See also: Custom Emoji

Endpoints​

Additional URLs associated with an actor, typically including sharedInbox for efficient delivery.

{
  "endpoints": {
    "sharedInbox": "https://example.com/inbox"
  }
}

F​

A collection of pinned posts, displayed prominently on an actor's profile. Mastodon extension.

Fediverse​

A portmanteau of "federation" and "universe." The network of interconnected servers using ActivityPub and related protocols.

See also: What is the Fediverse?

Federation​

The process of servers communicating with each other to share content and activities across the network.

See also: Understanding Federation

FEP (Fediverse Enhancement Proposal)​

Community-driven specifications for extending ActivityPub functionality.

See also: Fediverse Enhancement Proposals

Follow​

An activity expressing intent to subscribe to an actor's content. Typically requires an Accept response.

See also: Following and Followers

Followers Collection​

A collection containing actors who follow a given actor.

Following Collection​

A collection containing actors that a given actor follows.

G​

Group​

An Actor type representing a collective, such as a community or forum. Used by Lemmy for communities.

H​

Handle​

A user identifier in the format @username@domain, similar to email addresses.

Hashtag​

A tag starting with # used to categorize content. Implemented using the Hashtag type in the tag array.

See also: Mentions and Hashtags

HTTP Signatures​

A mechanism for authenticating HTTP requests using public key cryptography. Required for inbox delivery.

See also: HTTP Signatures Overview

I​

Inbox​

An endpoint where an actor receives activities from other servers. Every actor must have an inbox.

See also: Inbox Endpoint

Instance​

A single installation of Fediverse software running on a domain. Also called a "server."

IRI (Internationalized Resource Identifier)​

A generalized form of URI that supports Unicode characters. Used as identifiers in JSON-LD.

J​

JRD (JSON Resource Descriptor)​

The response format for WebFinger queries.

{
  "subject": "acct:user@example.com",
  "links": [...]
}

JSON-LD​

JavaScript Object Notation for Linked Data. The format used by ActivityStreams and ActivityPub for representing data.

See also: JSON-LD Security

K​

Key ID​

The identifier for a public key, used in HTTP Signatures to locate the key for verification.

keyId="https://example.com/users/alice#main-key"

L​

Like​

An activity expressing positive sentiment or appreciation. Equivalent to "favorite" on some platforms.

See also: Likes and Shares

Linked Data Signatures​

A method for cryptographically signing JSON-LD documents themselves (as opposed to HTTP requests).

Local-only​

Content that is not federated to other servers. Supported by some platforms as a visibility option.

M​

Mastodon​

The most popular ActivityPub implementation, a microblogging platform similar to Twitter.

See also: Mastodon, Mastodon Compatibility

Mention​

A reference to another actor within content, typically rendered as a link. Uses the Mention type in the tag array.

See also: Mentions and Hashtags

Move​

An activity used for account migration, indicating that an actor has moved to a new account.

See also: Account Migration

N​

NodeInfo​

A protocol for exposing server metadata, including software name, version, and statistics.

See also: NodeInfo Endpoint

Note​

The most common object type, representing a short text post. Used by Mastodon for toots/posts.

O​

Object​

A thing that activities act upon. Can be content (Note, Article, Image) or other entities.

See also: Object Types, Object Properties

OrderedCollection​

A Collection where the order of items is significant. Used for outboxes and timelines.

Organization​

An Actor type representing a formal organization, company, or institution.

Outbox​

An endpoint listing activities an actor has published. Used for fetching an actor's posts.

See also: Outbox Endpoint

P​

Page​

An object type used by Lemmy for posts (as opposed to Notes for comments).

Person​

The most common Actor type, representing an individual user account.

Poll​

Interactive voting content implemented using the Question type with oneOf or anyOf options.

See also: Polls

preferredUsername​

The local part of an actor's handle (without the domain).

Public Address​

The special URL https://www.w3.org/ns/activitystreams#Public indicating public visibility.

publicKey​

The RSA public key associated with an actor, used for HTTP Signature verification.

Q​

Question​

An object type representing a poll with voting options.

R​

Relay​

A server that redistributes activities to multiple instances, helping smaller servers discover content.

Reply​

A Note or other object that references another object via inReplyTo.

See also: Posts and Replies

S​

S2S (Server-to-Server)​

The ActivityPub protocol for servers to communicate with each other. The primary federation mechanism.

See also: Server-to-Server

Sensitive​

A boolean property indicating content should be hidden behind a warning. Used with summary for content warnings.

Service​

An Actor type representing an automated service or bot.

Shared Inbox​

A single inbox endpoint that receives activities for all actors on a server, reducing delivery overhead.

Signature​

See HTTP Signatures.

T​

Tag​

An array property containing Hashtags, Mentions, and Emoji referenced in content.

Thread​

A conversation consisting of a post and its replies, linked via inReplyTo.

Tombstone​

An object type representing deleted content, containing formerType and deleted timestamp.

{
  "type": "Tombstone",
  "formerType": "Note",
  "deleted": "2024-01-15T12:00:00Z"
}

Toot​

Mastodon's original term for a post (now officially called "post"). Technically a Note in ActivityPub.

U​

Undo​

An activity that reverses a previous activity, such as unfollowing or unliking.

Unlisted​

A visibility level where content is public but not shown in public timelines.

V​

Visibility​

The audience for content, determined by addressing. Common levels: public, unlisted, followers-only, direct.

W​

WebFinger​

A protocol (RFC 7033) for discovering information about actors using their handle.

See also: WebFinger Overview, WebFinger Implementation

Special Characters​

@context​

The JSON-LD property defining the vocabulary/namespace for a document.

@id​

The JSON-LD property for the unique identifier of an object. Often shortened to id in ActivityStreams.

@type​

The JSON-LD property for the type of an object. Often shortened to type in ActivityStreams.

Numeric​

202 Accepted​

The HTTP status code returned when an inbox successfully receives an activity for processing.

401 Unauthorized​

HTTP status indicating missing or invalid authentication (often signature verification failure).

403 Forbidden​

HTTP status indicating the request was understood but refused (e.g., blocked actor).

404 Not Found​

HTTP status indicating the requested resource doesn't exist.

410 Gone​

HTTP status indicating a resource was deleted. Used for tombstoned content.

See Also​