Ader, Clement, ah DAY, klay MAHN (1841-1925), a French electrical engineer, was a pioneer of powered flight. Ader built and piloted one of the earliest powered airplanes. He invented the French word for airplane, avion, from the initial letters of the phrase appareil volant imitant les oiseaux naturels, which means flying machine imitating natural birds. Aviation historians, however, consider Ader a controversial figure and believe that some of his claims were exaggerated.