Definition of reserve clause in US English:

reserve clause

noun

North American
  • 1Law a clause in a legal document reserving the right of certain parties to alter or repeal the terms of the document.

  • 2Sport a clause in a professional player's contract with a team, reserving the team's rights to the player once the contract has expired, preventing free movement between teams (now chiefly historical).

Origin

Mid 19th century.