It's defined in opencode docs, but it's an overall cross industry term for custom system prompt with it's own permissions:

https://opencode.ai/docs/agents/

Thanks for reference the docs. For me an agent is an entity that you can ask something and it talks to you and try to do what you asked to do.

In this case if you have a server with an endpoint you can run opencode when the endpoint is called and pass it the prompt. Opencode then think, plan and act accordingly to you request, possibly using tools, skills, calling endpoints,etc.

I'm still kind of confused, but opencode itself comes with several agents built-in, and you can also build your own. So what does it mean to use opencode itself as an agent?

Claude code too has build and plan agents.