Skill
A packaged set of instructions, and sometimes scripts, that an AI app loads when a task calls for it, so you do not repeat the same guidance every time.
A skill is usually a folder with a description file and any helper material. The app reads the short description first and loads the rest only when the task matches. That keeps the context-window free for everything else.
Think of it as a reusable procedure, such as how your team writes release notes. It is different from a system-prompt, which applies to every turn, because a skill is loaded on demand.
Only install skills from sources you trust. A skill can include code, and it shapes what the model does.