I.  Definitions
Air-report. A report from an aircraft in flight prepared in conformity with requirements for position, and operational and/or meteorological reporting.
Air-taxiing. Movement of a helicopter/VTOL above the surface of an aerodrome, normally in ground effect and at a ground speed normally less than 37 km/h (20 kt).
Note.— The actual height may vary, and some helicopters may require air-taxiing above 8 m (25 ft) AGL to reduce ground effect turbulence or provide clearance for cargo slingloads.

Air-to-ground communication. One-way communication from aircraft to stations or locations on the surface of the earth.
Air traffic. All aircraft in flight or operating on the manoeuvring area of an aerodrome.
Air traffic advisory service. A service provided within advisory airspace to ensure separation, in so far as practical, between aircraft which are operating on IFR flight plans.
Air traffic control clearance. Authorization for an aircraft to proceed under conditions specified by an air traffic control unit.
Note 1.— For convenience, the term “air traffic control clearance” is frequently abbreviated to “clear-ance” when used in appropriate contexts.
Note 2.— The abbreviated term “clearance” may be prefixed by the words “taxi”, “take-off”, “departure”, “en-route”, “approach” or “landing” to indicate the particular portion of flight to which the air traffic control clearance relates.

Air traffic control instruction. Directives issued by air traffic control for the purpose of requiring a pilot to take a specific action.
Air traffic control service. A service provided for the purpose of:
a)  preventing collisions:
1)  between aircraft, and
2)  on the manoeuvring area between aircraft and obstructions; and
b)  expediting and maintaining an orderly flow of air traffic.
Air traffic control unit. A generic term meaning variously, area control centre, approach control office or aerodrome control tower.
Air traffic service. A generic term meaning variously, flight information service, alerting service, air traffic advisory service, air traffic control service (area control service, approach control service or aerodrome control service).
Air traffic services airspaces. Airspaces of defined dimensions, alphabetically designated, within which specific types of flights may operate and for which air traffic services and rules of operation are specified.
Note.— ATS airspaces are classified as Class A to G as shown in Annex 11, Appendix 4.
Air traffic services reporting office. A unit established for the purpose of receiving reports concerning air traffic services and flight plans submitted before departure.
Note.— An air traffic services reporting office may be established as a separate unit or combined with an existing unit, such as another air traffic services unit, or a unit of the aeronautical information service.
Air traffic services unit. A generic term meaning variously, air traffic control unit, flight information centre or air traffic services reporting office.
Airway. A control area or portion thereof established in the form of a corridor.
ALERFA. The code word used to designate an alert phase.
Alerting service. A service provided to notify appropriate organizations regarding aircraft in need of search and rescue aid, and assist such organizations as required.
Alert phase. A situation wherein apprehension exists as to the safety of an aircraft and its occupants.
Allocation, allocate. Distribution of frequencies, SSR Codes, etc. to a State, unit or service. Distribution of 24-bit aircraft addresses to a State or common mark registering authority.
　
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