When you pay for an option, this prevents a person from revoking his offer before the expiration of the specified time.

Option

In finance, a contract that gives the buyer the right, but not the obligation, to buy or sell an asset at a predetermined price within a specified timeframe.

Revoking

The action of withdrawing, canceling, or annulling something previously granted, such as a privilege, right, or contract.

Offer

A proposal by one party to enter into a contract upon specified terms, subject to acceptance.