# Glossary

> The terms used throughout these docs, defined precisely.

_Source: https://aiprimetech.io/docs/resources/glossary/ · Home > Docs > Resources_

| Term | Meaning |
|---|---|
| **Token** | The unit models read and write, roughly ¾ of an English word. Billing is per million tokens. |
| **Input tokens** | Everything you send: system prompt, full message history, tool schemas, images. Grows every turn of a conversation. |
| **Output tokens** | What the model generates. Costs 4–5× input on every frontier model. |
| **Context window** | The maximum tokens a model can consider at once, input plus output. Exceeding it is an error, not silent truncation. |
| **Base URL** | The origin a client sends requests to. The single most common misconfiguration — see [Base URLs](/docs/getting-started/base-urls/). |
| **Messages API** | Anthropic's request format. `POST /v1/messages`, replies in a `content[]` array. |
| **Chat Completions** | OpenAI's request format. `POST /v1/chat/completions`, replies in `choices[]`. |
| **Streaming / SSE** | Server-sent events delivering the reply token by token instead of in one body. |
| **stop_reason** | Why generation ended: `end_turn`, `max_tokens`, `stop_sequence` or `tool_use`. |
| **Prompt caching** | Re-reading a stable prefix at a reduced input rate. See [Prompt caching](/docs/guides/prompt-caching/). |
| **Tool use** | The model requesting that you run a function, then continuing with the result. |
| **MCP** | Model Context Protocol — a standard way to expose tools to an agent. |
| **Credits** | Prepaid balance spent at per-token rates. Do not expire. |
| **Unlimited plan** | Flat-rate access for a fixed window, under fair-use limits, instead of per-token billing. |
| **Gateway** | A service that fronts one or more model providers behind a single API and key. |

- [Models](https://aiprimetech.io/docs/getting-started/models/) — Ids, context windows, rates
- [Pricing](https://aiprimetech.io/docs/billing/pricing/) — How credits work


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