A reusable instruction file that teaches an agent how to do one job well — the procedure, the tools, and what counts as done.
A skill is a folder with a markdown file describing a task in enough detail that an agent can repeat it: the steps, the conventions, the checks. The agent loads it only when the task comes up, which is the point — a hundred skills cost nothing until one is relevant.
The practical difference from a prompt is durability. A prompt lives for one conversation; a skill is committed to a repository, reviewed like code, and improved every time it fails. Most of what teams call “prompt engineering” is now skill authoring.