A mention in one document that points to another, so the agent can pull it into the context window only when the task calls for it. The unit progressive disclosure is built from.
Avoid: "reference" — too dry; doesn't convey that following it pulls more context in. "Portal" — too florid.
Usage:
"AGENTS.md is getting huge."
"Most of it should be context pointers, not content. Keep the always-on rules inline; turn the deploy runbook and the style guide into skills and leave a context pointer behind."