An autonomous reaction is one which a Sim does that isn't player defined. These can be reactions to the environment, for example, a smelly trash can, an unusual event, for example, a Sim dying, a bad memory, a missing object or simply something for a Sim to do if they haven't been told to do anything - providing that free will is on. This adds an extra dimension to the game, making the Sims unpredictable and somewhat free from complete player domination - however, a Sim will terminate most actions if the player either tells them to do so or tells them to do something else.