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]

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "arboretum":
   Japanese garden, afforestation, alpine garden, bed, bog garden,
   boondocks, border, botanical garden, bush, bushveld, chase,
   climax forest, cloud forest, dendrology, dry garden, flower bed,
   flower garden, forest, forest land, forest preserve, forestry,
   fringing forest, gallery forest, garden, garden spot, grape ranch,
   grapery, greenwood, hanger, herbarium, hortus siccus, index forest,
   jardin, jungle, jungles, kitchen garden, market garden,
   national forest, ornamental garden, palmetto barrens, paradise,
   park, park forest, pine barrens, pinetum, primeval forest,
   protection forest, rain forest, reforestation, rock garden,
   roof garden, scrub, scrubland, selection forest, shrubbery,
   shrubland, silviculture, sprout forest, stand of timber,
   state forest, sunken garden, tea garden, timber, timberland,
   tree veld, truck garden, vegetable garden, victory garden, vinery,
   vineyard, virgin forest, wildwood, wood, woodland, woods