Short answers, pulled from the story.
A free agent is a player or manager not under contract with any specific team, who is eligible to sign with any club or franchise. The term also applies to players who are currently under contract but are permitted to solicit offers from other teams.
The 1995 Bosman ruling by the European Court of Justice established the right of free agency for association football players across all EU member nations. It allowed players whose contracts had expired to move to another club without a transfer fee being paid between clubs.
The NFL's current free agency system was introduced on the 1st of March 1993. It replaced Plan B free agency, which had been in place from 1989 to 1992 before a 1992 jury found it violated antitrust laws.
An unrestricted free agent is free to sign with any team without conditions. A restricted free agent can negotiate with other clubs, but their original team has the right to match any new offer and retain the player, or receive draft-pick compensation if they decline to match.
Sol Campbell was released by Notts County in September 2009, just one month after joining the club on a free transfer. Because the transfer window had closed, he could not sign with another club immediately. He eventually signed with Arsenal in January 2010 during the winter transfer window.
Plan B free agency was a system active in the NFL from February 1989 to 1992 that allowed each team to protect up to 37 players from signing elsewhere. Eight players sued the NFL, and in 1992 a jury found Plan B violated antitrust laws, leading to its abolition.
Free agency became available in the National Women's Soccer League in 2023. Players must have at least six years of NWSL service to be eligible.