> ## Documentation Index
> Fetch the complete documentation index at: https://docs.world.org/llms.txt
> Use this file to discover all available pages before exploring further.

# Core Concepts

> Minimal concepts and vocabulary for integrating World ID.

World ID is designed for straightforward integration. This page covers only the core concepts most teams need before shipping.

## World ID

A user's self-custodial identity which lives in their Authenticator. When referring to the protocol, it's usually written as the *World ID Protocol*.

## Relying Party (RP)

A third-party application that wants to verify a user's credential. This is probably you.

## Credential

A signed attestation about a subject used to generate proofs. It includes issuer, subject, validity window, and claim commitments as defined in the [Credential's World ID 4.0 specs](https://docs.rs/world-id-primitives/latest/world_id_primitives/credential/struct.Credential.html)

## Issuer

An entity that issues credentials to users. In World ID, issuers are responsible for verifying a user's identity and signing their credential. Issuers must be registered in the `CredentialSchemaIssuerRegistry` to be recognized by the protocol.

## Proof Concepts

* **Action**: A developer-facing primitive that lets you put any app operation behind a unique-human gate. An app can have one or more actions depending on your use case.
* **Zero-Knowledge Proof (ZKP)**: A cryptographic method to prove that a statement is true without revealing any information about the statement itself. World ID uses ZKPs to prove that a user is verified without revealing the user's identity.
* **Nullifier**: A component of the World ID ZKP; a unique identifier for a combination of a user, `app_id`, and `action`.
* **Signal**: A component of the World ID ZKP; data attached to the proof that cannot be tampered with. An example may be a user's choice for an election.

# Legacy Concepts (World ID 3.0)

* **Identity Commitment**: A hash of a user's secret identity nullifier and trapdoor, which is inserted into the Merkle Tree. This is no longer used in World ID 4.0, but you may see it referenced in older documentation.
* **App ID**: The ID of your app that is assigned in our [Developer Portal](https://developer.worldcoin.org/). In World ID 4.0, this is now referred to as **RP ID**.
* **Merkle Root**: A component of the World ID ZKP; The root of the [Merkle Tree](https://en.wikipedia.org/wiki/Merkle_tree) that identity commitments are inserted to. While 4.0 still uses Merkle Trees, the root is now encoded within the received ZKP, so you don't need to worry about handling it.