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