The Collaborative International Dictionary of English v.0.48:

Tern \Tern\ (t[~e]rn), n. [Dan. terne, t[ae]rne; akin to Sw.
   t[aum]rna, Icel. [thorn]erna; cf. NL. sterna.] (Zool.)
   Any one of numerous species of long-winged aquatic birds,
   allied to the gulls, and belonging to Sterna and various
   allied genera.
   [1913 Webster]

   Note: Terns differ from gulls chiefly in their graceful form,
         in their weaker and more slender bills and feet, and
         their longer and more pointed wings. The tail is
         usually forked. Most of the species are white with the
         back and wings pale gray, and often with a dark head.
         The common European tern (Sterna hirundo) is found
         also in Asia and America. Among other American species
         are the arctic tern (Sterna paradisaea), the roseate
         tern (Sterna Dougalli), the least tern (Sterna
         Antillarum), the royal tern (Sterna maxima), and the
         sooty tern (Sterna fuliginosa).
         [1913 Webster]

   Hooded tern. See Fairy bird, under Fairy.

   Marsh tern, any tern of the genus Hydrochelidon. They
      frequent marshes and rivers and feed largely upon insects.


   River tern, any tern belonging to Seena or allied genera
      which frequent rivers.

   Sea tern, any tern of the genus Thalasseus. Terns of this
      genus have very long, pointed wings, and chiefly frequent
      seas and the mouths of large rivers.
      [1913 Webster]