In air traffic control, what does the term 'approach clearance' specifically allow an aircraft to do?

The term 'approach clearance' specifically allows an aircraft to commence its instrument approach procedure. This clearance is a critical element in the air traffic control process, as it signifies that the aircraft has been given the authority to begin the approach to the airport, typically transitioning from en-route flight to the landing phase. The clearance ensures that the aircraft is following a designated flight path, which enables the controller to maintain safe separation from other aircraft. Obtaining approach clearance is essential for ensuring that the aircraft can operate safely and efficiently within the controlled airspace as it prepares for landing.

This is distinct from activities such as taxiing, taking off from the runway, or changing altitude just before landing, all of which are governed by different clearances. Therefore, approach clearance is specifically tied to the execution of an approach procedure rather than other phases of flight.