term:

Zipper Clause

zipper clause n

[from the comparison of such a clause to the closing action of a zipper]
: a clause in a labor contract indicating that the agreement is an exclusive and complete expression of consent NOTE: A zipper clause may constitute a waiver of the right to bargain over issues negotiated in or outside the terms of a contract until its expiration.

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.