Definitions of zipper clause
a term in an employment contract to stop the employee from demanding any changes to the contract once it has been agreed; a type of merger or integration clause (=a clause in a contract that states that this is a complete statement of the agreement in its final form, and that any matters discussed, either orally or in writing, during the drafting of the contract that are not included do not form part of the contract)
Management will likely propose a zipper clause, common in collective bargaining agreements.