Eligibility for the NBA Draft

The NBA draft is an annual event in which the 30 franchises in the National Basketball Association select new players for their teams. Eligibility rules for prospective players have changed several times during the history of the league. No player may sign with the NBA until he has been eligible for at least one draft. The rule has produced one-and-done players that play college basketball for one year before declaring for the draft.

In the earliest days of the NBA, three players entered the NBA without having played in college (although one of them did not enter the league until he was 39 years old). However, the league eventually established a rule that "a player could not make himself available" for the draft until four years after his high school graduation.

The first major challenge to the NBA's eligibility rules came from Spencer Haywood. He graduated from high school in 1968, at a time when college seniors were not allowed to play varsity sports for NCAA member schools. He played 3 years at a Colorado junior college, followed by a season at the University of Detroit. After the 1970–71 season, he left college for the NBA's rival at the time, the ABA, which had no rule restricting college underclassmen from entering the league, and had a spectacularly successful rookie season with the Denver Rockets (the predecessor to today's Denver Nuggets), being named the ABA's Rookie of the Year and MVP. Near the end of the season, he turned 21; shortly after its end, he repudiated his contract with the Rockets, claiming he had been defrauded. Haywood then signed a contract with the Seattle SuperSonics, which put him and the Sonics on a collision course with the NBA, as he was only three years removed from his high school graduation.

...
Wikipedia