offshore airport; airport built on the sea
Refers to an airport constructed on reclaimed land or an artificial island in the sea. The English gloss 'seadrome' is rare and technical; 'offshore airport' is more common.
Kansai International Airport is Japan's first full-scale offshore airport.
空港 is the general term for any airport, while 海上空港 specifically refers to an airport built on the sea.
Compound of 海上 (かいじょう, 'on the sea') and 空港 (くうこう, 'airport').