The freedom of an uncontracted athlete to sell his services to the highest bidder. In North American sports, it didn't happen until the mid-70s - athletes (including Curt Flood, who gave up his career over contractual issues) and agents successfully challenged the "reserve clause" that bound players to their teams. It led to exponential growth in salaries, which continues, and the replacement of "poor" millionaire owners (like Charles Finley) by billionaires and media conglomerates.