What is the opening at the top of a hot-air balloon’s envelope called?

The opening at the top of a hot-air balloon’s envelope is controlled by a parachute valve.

This circular fabric valve is fitted into the crown of the envelope and can be opened by the pilot using a line that runs down toward the basket. Pulling the line allows hot air to escape, reducing lift and helping the pilot descend or make a controlled landing.

Despite its name, the valve is not a separate parachute used to slow the entire balloon. It is a circular panel that closes again when the control line is released. The pilot can also briefly use it to adjust altitude during flight.

The valve must be handled carefully: opening it too far or for too long can produce a rapid descent. The term crown valve is also used in ballooning, but parachute valve is the standard answer.