What does "free agent" mean?

A person not under compulsion or constraint, not responsible to any authority for his or her actions. For example, After he quit his job, he decided to pursue the same line of work as a free agent. Originally used to describe a person subject to the philosophic concept of free will (as opposed to determinism), this expression was first recorded in 1662. Later it was extended to mean “someone not under obligation to an authority.”

A professional athlete who is free to sign a contract with any team. For example, After he was released from the Yankees, he was a free agent and could shop around for the team that offered the most money. [Second half of 1900s]