The Collaborative International Dictionary of English v.0.48:

Arboretum \Ar`bo*re"tum\, n.; pl. Arboreta. [L., a place grown
   with trees.]
   A place in which a collection of rare trees and shrubs is
   cultivated for scientific or educational purposes.
   [1913 Webster]

WordNet (r) 3.0 (2006):

arboretum
    n 1: a facility where trees and shrubs are cultivated for
         exhibition [syn: arboretum, botanical garden]