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
    n 1: any of various small insectivorous American birds chiefly
         olive-grey in color