# Glossary

URL: https://docs.bithuman.ai/resources/glossary

> The words used across the bitHuman docs, what each one means, and the older names you may still meet in code, files and saved links.

What each term means here, and the page that owns it.

## Models and avatars

| Term | Meaning |
|---|---|
| **Essence 2** (`essence-2`) | The model that renders a photoreal person from one portrait. [Essence 2](https://docs.bithuman.ai/models/essence-2) |
| **Expression 2** (`expression-2`) | The model that renders any character from one portrait. [Expression 2](https://docs.bithuman.ai/models/expression-2) |
| **Essence 2 Max** | Essence 2 Max is available on the Enterprise plan only. Contact sales to enable it. [Models](https://docs.bithuman.ai/models) |
| **Essence 1, Expression 1** (`essence-1`, `expression-1`) | The first generation of the models. [First generation](https://docs.bithuman.ai/models/first-generation) |
| **Agent** | An avatar together with its persona, voice and knowledge, created in the bitHuman app or with the [Agents API](https://docs.bithuman.ai/api/agents). |
| **Agent code** | The identifier of an agent, for example `A23WJF0199`. [Agent codes](https://docs.bithuman.ai/models/avatar-file#agent-codes) |
| **Sample avatars** | Public agents anyone can use without an account: `sofia-ramirez` (Essence 2) and `wise-pup` (Expression 2). [Showcase list](https://api.bithuman.ai/v1/models/showcase) |
| **Avatar file** (`.imx`) | One agent's model files in one container, downloaded once and rendered on your hardware. [The avatar file](https://docs.bithuman.ai/models/avatar-file) |
| **Persona** | The agent's system prompt: who it is and how it answers. [Persona](https://docs.bithuman.ai/build/persona) |
| **Gestures** | A named motion (wave, nod, clap) an avatar plays when your code asks; the API calls them `dynamics`. [Gestures](https://docs.bithuman.ai/build/gestures) |
| **Idle** | The motion an avatar shows between replies, so it never freezes. |

## Where it runs

| Term | Meaning |
|---|---|
| **Renders / runs** | The avatar *renders* (on the device, in the browser, on your server or in the bitHuman cloud); the conversation *runs* (in your stack, in the CLI's local conversation brain, or on bitHuman's servers). |
| **On the device** | The avatar renders on the user's iPhone, iPad, Mac or Android phone. [On the device](https://docs.bithuman.ai/deploy/on-device) |
| **CPU only (no GPU)** | The avatar renders on a standard Linux PC with no graphics card. [CPU only](https://docs.bithuman.ai/deploy/cpu) |
| **Your servers** | You run the CLI, the Python SDK or the LiveKit plugin on machines you control. [Your servers](https://docs.bithuman.ai/deploy/self-hosted) |
| **bitHuman cloud** | The avatar renders on bitHuman's servers and streams to any screen. [bitHuman cloud](https://docs.bithuman.ai/deploy/cloud) |
| **Fully offline** | A license for realtime avatars with no internet connection. [Fully offline](https://docs.bithuman.ai/deploy/offline) |
| **File rendering** | Turning an audio file into a talking-avatar video with `bithuman render` or the Python SDK. It signs in online; it is not the [offline license](https://docs.bithuman.ai/deploy/offline). [Talking video](https://docs.bithuman.ai/build/talking-video) |
| **WebGPU** | The browser graphics interface the web embed uses to render the avatar in the tab (`render=local`). [Web](https://docs.bithuman.ai/platforms/web) |
| **Local conversation brain** | The CLI mode (`BITHUMAN_LOCAL=1`) that runs speech recognition, the language model and the voice on a Mac or Linux PC. [Local conversation brain](https://docs.bithuman.ai/platforms/cli/local-brain) |
| **× real time** | Seconds of avatar video rendered per second. At 1.0× or more an avatar holds a live conversation. [Performance](https://docs.bithuman.ai/performance) |

## Credentials and billing

| Term | Meaning |
|---|---|
| **API secret** | The one credential for every surface, read from `BITHUMAN_API_SECRET`; REST sends it in the `api-secret` header. [Your API secret](https://docs.bithuman.ai/start/api-secret) |
| **Embed token** | A short-lived token your server mints so a web page can open a private agent without the secret. [Embedding](https://docs.bithuman.ai/api/embedding) |
| **Runtime token** | A short-lived token for a LiveKit room or a download, minted with the API secret. [Authentication](https://docs.bithuman.ai/api/authentication) |
| **Credits** | What usage is paid in. [Pricing and credits](https://docs.bithuman.ai/pricing) |
| **Active session time** | What realtime usage bills: the time a session is open, talking or idle, to the second. [Pricing](https://docs.bithuman.ai/pricing) |
| **Creator plan** | The plan API and SDK use requires, or a higher one. [Plans](https://docs.bithuman.ai/pricing) |

## Older names

The model names retired over time, the older file extensions and library names, and what each means today, are in one table: [Naming & migration](https://docs.bithuman.ai/models#naming--migration). The SDK and API names below are older spellings you may still meet:

| Older name | What to use now |
|---|---|
| `BITHUMAN_API_KEY` | `BITHUMAN_API_SECRET`. The deprecated alias is still read, with a warning, until CLI 3.0 and bithuman 4.0. |
| `POST /v1/realtime/ephemeral-token` (`ek_…` tokens) | Retired; connect through the [realtime relay](https://docs.bithuman.ai/api/realtime). |
| `bithuman.offline`, `render_offline` | Deprecated; use `bithuman.open(path).render(audio, out_mp4=...)` ([Python](https://docs.bithuman.ai/platforms/python)). |
| `bitHumanKit` | A legacy Swift package, not the current one; use the [Swift package](https://docs.bithuman.ai/platforms/ios) products `Expression2` and `Essence2Kit`. |

## Next

- [FAQ](https://docs.bithuman.ai/resources/faq) · [Models](https://docs.bithuman.ai/models) · [Deploy](https://docs.bithuman.ai/deploy)