Coding Agent, or Agent CLI – a command-line application, used to launch and manage agent instances (e.g. Opencode, Claude Code, Copilot CLI, Gemini CLI, etc)
Agent Definition – a file (Markdown/JSON) that describes how an agent behaves; serves as a blueprint from which instances are created (like a class in Java)
Agent Instance – a running process created from an Agent Definition (like an object instantiated from a class in Java).
Agent / AI Agent – avoid; use the specific term (CLI, Definition, or Instance) depending on what you mean; not to forget that “Agent” can also mean the CI pipeline agent or a human agent