Autonomy originates from the Ancient Greek autonomia (independence), which is from autonomos ("independent, living by one’s own "laws").   It historically refers to a state or individual having the right to self-government or independence.  Modern usage spans political sovereignty to individual self-determination. 

According to the Oxford English Dictionary (OED), the earliest known use of the noun "autonomy" in English is from the late 1500s,  specifically in 1591 in the writing of A. Colynet.  The term refers to self-government or independence and is a borrowing from Greek (autonomia).  Additionally, the OED notes the adjective autonomous appeared later, in the late 1600s (specifically 1673, by Richard Baxter).   Originally, the term referred primarily to the "autonomous condition, power or right of self-government" of states, communities, or city-states.   The application of autonomy to individual persons (personal autonomy) became more common in English in the early 1800s, gaining prominence through Kantian metaphysics. 

Key Concepts in Autonomy:  
Kantian Autonomy: Rational action based on self-legislated moral laws. 
Personal Autonomy: The capability to make intentional decisions and direct one's own life. 
Self-Rule: Freedom from external control (duress, coercion) and internal constraints (lack of capacity).

14 minutes ago, drpaladin said:

In life, most of us seek out automony as an ideal.

There are many people who believe they achieve autonomy through sovereign citizenship. 

Character autonomy—a character’s ability to make choices, drive the plot, and act on their own motivations—is essential for creating engaging, believable stories.  It transforms characters from passive plot devices into active participants, fostering reader empathy, strengthening character arcs, and enhancing narrative tension by ensuring choices have authentic consequences. 

Drives the Plot (Agency): Autonomous characters cause the story to happen through their decisions rather than being moved around by the author, ensuring the plot feels natural rather than forced. 
Boosts Reader Engagement: Readers connect with characters who take charge of their destiny, fight for their goals, and face consequences. A character with strong desires and the power to pursue them makes the story more immersive. 
Creates Believable Development: When characters act according to their own personalities and motivations, their growth feels earned and organic. 
Enhances Conflict: Autonomy allows characters to push back against external pressures (antagonists, circumstances), which drives the tension necessary for compelling narratives. 
Prevents "Flat" Characters: Giving characters the space to think for themselves, take up space, and show personal motivations prevents them from appearing as one-dimensional stereotypes. 

Autonomy is what brings a character to life, moving them beyond their initial design to become the true, active heart of the story.