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Wiktionary
fly-in

n. 1 an informal gathering of private pilots and their aircraft at a prearranged airfield. 2 (context roofing English) method of application for roll materials by which the dry sheet is set into the bitumen or adhesive applied to the roof surface.

Wikipedia
Fly-in

A fly-in is a pre-arranged gathering of aircraft, pilots and passengers for recreational and social purposes.

Fly-ins may be formally or informally organised, members of the public may or may not be invited, the gathering may be at an airport or in a farmer's field.

Fly-ins can be aimed at specific aircraft classes, such as taildraggers, warbirds, experimental aircraft or specific aircraft models. They may be organized by a national organization, such as the Canadian Owners and Pilots Association or the Experimental Aircraft Association, the airport owner or authority, a flying club, an aircraft type club or by a group of friends meeting perhaps for a barbecue and to socialize.

The term "fly-in" is not formally defined in the aviation legislation of many countries and it may refer to a range of events, while in others it has a specific legal meaning. For example in Canada fly-ins may not legally include air displays or competitive flying.