Airport

"What's an airport again?" ~ Andy, Kanbei's Error

Airports are a property for building and maintaining air units.

In-depth
Airports give three terrain stars.

They can be used to build and repair air units, generate income (usually 1000 G per day) and provide terrain cover.

In Advance Wars 2: Black Hole Rising, Lash's ground units get a 30% attack boost on airports. This is reduced to 15% in Advance Wars: Dual Strike. Kindle's ground and air units in Advance Wars: Dual Strike get a 40% attack boost while on airports.

In Days of Ruin, bases provide a hiding place for ground units, and can only be revealed either when an enemy unit moves past the city or if they're in the vision range of one of Lin's ground units when she uses her CO Power.