Roger Oriol

An agent is something that acts in an environment; it does something. Agents include worms, dogs, thermostats, airplanes, robots, humans, companies, and countries. Artificial Intelligence: Foundations of Computational Agents, 3rd edition by David L. Poole and Alan K. Mackworth, Cambridge University Press 2023

This is the definition I personally like the best for what agents are in the context of AI. Since the overuse of this word has left some of us confused on what it actually means, I would say that any application that uses AI and has the ability to act on its environment, through tools or function calling, is an agent.