A teachable capability bundled as a unit — instructions and resources for doing one task well, kept in the environment until a context pointer pulls it into the context window for the task at hand. The unit of progressive disclosure in a harness.
Avoid: "tool" — a tool is what the agent calls; a skill is instructions it reads.
Usage:
"Where should I put the deploy runbook?"
"As a skill — the agent loads it only when the task involves deploys. In AGENTS.md it'd burn tokens on every turn for something we use weekly."