An escalator clause is a clause in a lease

Lease

A lease is a contractual arrangement calling for the lessee to pay the lessor for use of an asset. A rental agreement is a lease in which the asset is tangible property...

 or contract

Contract

A contract is an agreement entered into by two parties or more with the intention of creating a legal obligation, which may have elements in writing. Contracts can be made orally. The remedy for breach of contract can be "damages" or compensation of money. In equity, the remedy can be specific...

 that guarantees a change in the agreement price once a particular factor beyond control of either party affecting the value has been determined.

Escalator clauses are quite common in construction contracts to cover unexpected costs due to fluctuations in the prices for raw materials, fuel, and labor during the course of the construction project.