tern, common name for a sea bird of the Old and New Worlds, smaller than the related gull. Because of their graceful flight and their long pointed wings and forked tails, some terns are called sea swallows. They plunge headlong into the water to catch small fish. The arctic tern migrates from the arctic to the antarctic. American terns include the common, least, Forster's, noddy, sooty, roseate, and royal terns, all of the genus Sterna. Terns are classified in the phylum Chordata, subphylum Vertebrata, class Aves, order Charadriiformes, family Laridae.