The definition, in one sentence
An AI coding agent is a system that takes a goal, breaks it into steps, and does them for you. It reads and writes code, runs commands, and checks the results. It keeps going until the goal is done or it needs your help. The key word is 'does'. A chatbot answers your questions; an agent takes action. It does not just suggest the next line. It works toward a result through many steps, in your real project.