The Collaborative International Dictionary of English v.0.48:

lovebird \lovebird\ n.
   A small Australian parakeet (Melopsittacus undulatus)
   usually light green with black and yellow markings in the
   wild but bred in many colors; also called the budgie.

   Syn: budgerigar, budgereegah, budgerygah, budgie, grass
        parakeet, shell parakeet, Melopsittacus undulatus.
        [WordNet 1.5]

   2. small African parrot noted for showing affection for their
      mates.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

lovebird
    n 1: small Australian parakeet usually light green with black
         and yellow markings in the wild but bred in many colors
         [syn: budgerigar, budgereegah, budgerygah, budgie,
         grass parakeet, lovebird, shell parakeet,
         Melopsittacus undulatus]
    2: small African parrot noted for showing affection for their
       mates