noun
biplane
A fixed-wing aircraft with two main wings stacked one above the other.
第一次世界大戦では多くの複葉機が使われた。
Many biplanes were used in World War I.
飛行機 is the general term for airplane; 複葉機 specifies the biplane configuration.
複葉 (ふくよう, 'compound leaf' or 'multiple wings') + 機 (き, 'machine, aircraft'). The term directly describes the two-wing structure.