Terceira (tĕrsāˈrə) [key], island (1991 pop. 59,248), 153 sq mi (396 sq km), in the N Atlantic, one of the central Azores, Portugal. The chief town is Angra do Heroísmo, which gives its name to the district. Grains, cattle, and embroidery are exported, and there is a growing tourist trade. There is a NATO air base there.