Aircraft on ground

Aircraft on ground or AOG is a term in aviation maintenance indicating that a problem is serious enough to prevent an aircraft from flying. Generally there is a rush to acquire the parts to put the aircraft (A/C) back into service, and prevent further delays or cancellations of the planned itinerary. DHL estimates that an AOG can cost as much 925,000 Euro per day. AOG applies to any aviation materials or spare parts that are needed immediately for an aircraft to return to service. AOG suppliers refer qualified personnel and dispatch the parts required to repair the aircraft for an immediate return to service. AOG also is used to describe critical shipments for parts or materials for aircraft "out of service" or OTS at a location. 
Mitigation of AOG status.
When an aircraft "goes AOG" and materials required are not on hand, parts and personnel must be driven, flown, or sailed to the location of the "grounded A/C". Usually the problem is escalated through an internal AOG Desk, then the Manufacturer's AOG Desk, and finally competitors' AOG desks. All major air carriers have an "'AOG Desk". This desk is manned 24/7 by personnel trained in purchasing, hazardous materials shipping, and parts manufacturing / acquisition processes.
AOG personnel are trained to "loan" or "borrow" spare parts from other air carriers, per FAA/EASA etc. regulations. AOG personnel work in conjunction with their carriers' maintenance operations department, supporting aircraft maintenance with all parts or material requests very rapidly. There are two ways of achieving this: by local engineering support or by support flown out base.
Insurance policies are available that cover the costs that aircraft owners or operators may incur by having an aircraft out of service.