The Collaborative International Dictionary of English v.0.48:

Greenhouse \Green"house`\, n.
   A house in which tender plants are cultivated and sheltered
   from the weather.
   [1913 Webster]

WordNet (r) 3.0 (2006):

greenhouse
    adj 1: of or relating to or caused by the greenhouse effect;
           "greenhouse gases"
    n 1: a building with glass walls and roof; for the cultivation
         and exhibition of plants under controlled conditions [syn:
         greenhouse, nursery, glasshouse]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "greenhouse":
   alcove, arbor, bower, cloche, cold frame, conservatory, coolhouse,
   forcing bed, forcing house, forcing pit, gazebo, glasshouse,
   hotbed, hothouse, kiosk, lathhouse, nursery, orangery, pergola,
   pinery, retreat, seedbed, summerhouse