Source : Webster's Revised Unabridged Dictionary (1913)

Arbor \Ar"bor\, n. [Written also arbour.] [L., a tree, a beam.]
   1. (Bot.) A tree, as distinguished from a shrub.

   2. [Cf. F. arbre.] (Mech.)
      (a) An axle or spindle of a wheel or opinion.
      (b) A mandrel in lathe turning. --Knight.

   {Arbor Day}, a day appointed for planting trees and shrubs.
      [U.S.]

Source : WordNet®

Arbor Day
     n : a day designated for planting trees