The Collaborative International Dictionary of English v.0.48:

Herbarium \Her*ba"ri*um\, n.; pl. E. Herbariums, L.
   Herbaria. [LL., fr. L. herba. See Herb, and cf. Arbor,
   Herbary.]
   1. A collection of dried specimens of plants, systematically
      arranged. --Gray.
      [1913 Webster]

   2. A book or case for preserving dried plants.
      [1913 Webster]

WordNet (r) 3.0 (2006):

herbarium
    n 1: a collection of dried plants that are mounted and
         systematically classified for study

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "herbarium":
   Japanese garden, alpine garden, arboretum, bed, bog garden, border,
   botanical garden, dry garden, flower bed, flower garden, garden,
   garden spot, grape ranch, grapery, hortus siccus, jardin,
   kitchen garden, market garden, ornamental garden, paradise,
   pinetum, rock garden, roof garden, shrubbery, sunken garden,
   tea garden, truck garden, vegetable garden, victory garden, vinery,
   vineyard