The Collaborative International Dictionary of English v.0.48:

Greenlet \Green"let\, n.
   1. (Zool.) One of numerous species of small American singing
      birds, of the genus Vireo, as the solitary, or
      blue-headed (Vireo solitarius); the brotherly-love (V.
      Philadelphicus); the warbling greenlet (V. gilvus); the
      yellow-throated greenlet (V. flavifrons) and others. See
      Vireo.
      [1913 Webster]

   2. (Zool,) Any species of Cyclorhis, a genus of tropical
      American birds allied to the tits.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vireo \Vir"e*o\, n. [L., a species of bird.] (Zool.)
   Any one of numerous species of American singing birds
   belonging to Vireo and allied genera of the family
   Vireonidae. In many of the species the back is greenish, or
   olive-colored. Called also greenlet.
   [1913 Webster]

   Note: In the Eastern United States the most common species
         are the white-eyed vireo (Vireo Noveboracensis), the
         red-eyed vireo (Vireo olivaceus), the blue-headed, or
         solitary, vireo (Vireo solitarius), the warbling
         vireo (Vireo gilvus), and the yellow-throated vireo
         (Vireo flavifrons). All these are noted for the
         sweetness of their songs.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Vireo solitarius
    n 1: of eastern North America having a bluish-grey head and
         mostly green body [syn: solitary vireo, Vireo
         solitarius]