A rational agent is an entity which has a utility function, forms beliefs about its environment, evaluates the consequences of possible actions, and then takes the action which maximizes its utility. They are also referred to as goal-seeking. The concept of a rational agent is used in economics, game theory, decision theory, and artificial intelligence.
More generally, an agent is anything that can be viewed as perceiving its environment through sensors and acting upon that environment through actuators.1
There has been much discussion as to whether certain AGI designs can be made into mere tools or whether they will necessarily be agents which will attempt to actively carry out their goals. Any minds that actively engage in goal-directed behavior are potentially dangerous, due to considerations such as basic AI drives possibly causing behavior which is in conflict with humanity's values.
In Dreams of Friendliness and in Reply to Holden on Tool AI, Eliezer Yudkowsky argues that, since all intelligences select correct beliefs from the much larger space of incorrect beliefs, they are necessarily agents.