Glossary

The actors and shared terms every spec uses. A spec’s own Terms section defines only terms local to that spec. Add an actor here, not in a spec (SPEC-038).

This file names things; it does not constrain them. Under SPEC-072 every obligation belongs to a requirement in the spec that owns the capability, so an entry here says what a term refers to and points at the spec that states its rules. A property worth enforcing, such as how sequence ids are ordered or how a message hash is computed, is a requirement there and not a sentence here.

Actors

Actor	Meaning
The backend	The self-hosted XMTP server: the gRPC services in the xmtp.backend.v1 package and the process that serves them.
The client	libxmtp: the Rust library that holds keys, MLS state, and the local database, on native and wasm targets.
An SDK	A language binding on top of the client: Kotlin, Swift, or TypeScript. Specs describe SDK behaviour abstractly, not by method name.
An app	The developer’s program that uses an SDK.
An operator	The party that deploys and configures a backend.
A member	An inbox that belongs to a group. “Another member” is a member other than the one whose client is acting.
A peer installation	Another installation of the same inbox as the acting client.
A sender	The installation that published an envelope.
A recipient	An installation the backend delivers an envelope or a push notification to.
The storage target	The object storage an operator configures for remote attachments. It accepts the upload requests the backend signs and serves the stored ciphertext at its download URLs (ATCH).
A validator	Whichever party checks a payload, when the obligation is the same for the backend and the client. Use a named actor when only one of them is bound.

Terms

Term	Meaning
Inbox	The identity a user acts as, named by an inbox id. Derivation and length are stated by IDENT.
Installation	One client instance with its own key pair, named by an installation key. An inbox has many. Stated by IDENT.
Identifier	An external account bound to an inbox: an Ethereum address, a smart contract wallet, or a passkey.
Envelope	One unit of client data the backend stores: a group message, a Welcome, a key package, an identity update, or a commit-log entry.
Topic	The routing key of an envelope. Its layout is stated by TOPIC.
Sequence id	The integer the backend assigns to a stored envelope. Its range, ordering, and uniqueness are stated by API.
Cursor	A client’s position on one topic. Its meaning and bounds are stated by API.
Message hash	The digest that identifies a stored envelope. Its algorithm and role in idempotency are stated by API.
Group	An MLS group: a conversation named by a group id. A DM, a sync group, and a one-shot group are groups with a conversation type.
DM	A group between exactly two inboxes with the fixed DM policy. Owned by the DMS spec.
Epoch	The MLS epoch of a group. Every commit advances it.
Commit	An MLS message that changes a group’s state and advances its epoch.
Proposal	An MLS message that proposes a change for a later commit.
Welcome	The MLS message that adds an installation to a group, inline or through a welcome pointer.
Key package	An installation’s published MLS credential and keys that let another member add it to a group.
Identity update	A signed change to an inbox’s association log. Owned by the IDENT spec.
Handle	A public name, such as nick or nick#2341, that an inbox claims with a ClaimHandle action in its association log. Its grammar and the claim rules are stated by IDENT section 10.
Name part	The text of a handle before its #, or the whole handle when it has no #.
Display handle	The handle an inbox is shown by: the last of the handles in its association state (IDENT-082). An inbox that has claimed no handle has none.
Commit-log entry	A signed record of a commit that a client publishes for fork detection. Owned by the FORK spec.
Consent	An inbox’s or conversation’s state of allowed, denied, or unknown. Owned by the CONS spec.
Content type	The typed encoding of a message payload. Owned by the CTYPE spec.
Remote attachment	The description of a file a message carries by reference: the URL of an encrypted object, with the key material and digest to fetch and decrypt it. Its encoding is owned by CTYPE; its storage, upload, and download by ATCH.