escalator clause

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A provision in an agreement or regulation giving a party or beneficiary the benefit of an increase in payment, or an excuse from payment of an additional amount, where some specified change in financial conditions occurs, such as an increase in cost of living.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun law A clause in a lease or contract that guarantees a change in the agreement price once a particular factor beyond control of either party affecting the value has been determined.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a clause in a contract that provides for an increase or a decrease in wages or prices or benefits etc. depending on certain conditions (as a change in the cost of living index)