Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

arboretum
     n : a facility where trees and shrubs are cultivated for
         exhibition [syn: {botanical garden}]
     [also: {arboreta} (pl)]