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Verified against AS2 Core & Vocabulary (W3C REC 2017), the published AS2 JSON-LD context, ActivityPub REC, Mastodon v4.7.3 source (context_helper.rb, account.rb, json_ld_helper.rb), RFC 7033, FEP index (codeberg), Mastodon v4.7.3 locales on 2026-10-15.

Glossary

Terms used in ActivityPub, ActivityStreams and the Fediverse, with links to the pages that cover them in depth.

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 — there is no Actor type itself in AS2.

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​

Fetching content you missed: an account's older posts after a new follow, or the rest of a conversation (replies, parents) when you only received part of a thread. See FEP-7458 (replies collection), FEP-7888 (context) and FEP-f228 (conversation backfill), all drafts.

Block​

An activity by which an actor blocks another. Enforcement is up to each server: the blocking user's server stops showing and accepting the blocked actor's interactions, and Mastodon also delivers the Block so the remote server can hide content. Public posts can still be fetched unless the server requires signed fetches (Authorized Fetch).

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. The warning text goes in summary; Mastodon also sets the sensitive extension to 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, signed along with the request by HTTP Signatures (draft-cavage). RFC 9421 signatures use the Content-Digest header instead.

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. ActivityPub itself doesn't mandate an authentication method, but in practice every major server requires signed inbox deliveries: draft-cavage signatures everywhere, and RFC 9421 increasingly (Mastodon 4.5+).

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​

An older, now-deprecated method for signing JSON-LD documents themselves (as opposed to HTTP requests). Mastodon advises against implementing it, and a JSON-LD named-graph bypass (CVE-2026-46349) affected implementations in 2026. Its successor is Object Integrity Proofs (FEP-8b32).

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. Receivers must also accept the compact forms as:Public and Public. It is never a delivery target.

publicKey​

The key (usually RSA) an actor publishes for HTTP Signature verification, from the W3C Security Vocabulary. Newer keys such as Ed25519 are published in assertionMethod (FEP-521a); see Keys and Identity.

Q​

Question​

An IntransitiveActivity type, used as the object type for polls: options go in oneOf (single choice) or anyOf (multiple choice), never both.

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 kept out of public timelines. Mastodon now labels it "Quiet public" and also keeps it out of search, trending and hashtag feeds. In ActivityPub it is addressed with Public in cc instead of to.

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​