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A ground stop is an air traffic control measure that slows or halts the flow of inbound aircraft into a given airport. In other words, a ground stop is the halting of departing aircraft destined for one particular airport or for a specific geographic area. 
For example, if a ground stop is called for Miami International Airport, aircraft departing for Miami from other airports will not depart until such time that the ground stop in Miami is lifted. This allows, in this example, for Miami to deal with the task at hand preparing for arriving aircraft once the ground stop is lifted.
Flights already en route may be diverted to other cities, while those that have not departed their airport of origin will be delayed or cancelled. Airlines are required to manage their aircraft at all airports to minimize the impact to passengers affected by the ground stop.
On September 11, 2001, the U.S. FAA issued a ground stop order for the entire United States, as a precaution against possible additional incoming terrorist attacks by plane. In addition to grounding international flights which had not yet departed, flights already in the air were either re-called (if less than half-way) or diverted to airports in other countries, mostly Canada. The ground stop was lifted on September 13, when departures from airports within the US also resumed.
In January 2014, Nav Canada issued a ground stop for Toronto Pearson due to cold weather. "The airport authority issued a Tweet just after 7 a.m. saying the extreme cold is causing "equipment freezing and safety issues for employees."
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- Pearson airport delays: What you need to know, CBC.ca, January 7, 2014; retrieved January 7, 2014.
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