1. [n]  a framework that supports climbing plants; "the arbor provided a shady resting place in the park"
2. [n]  any of various rotating shafts that serve as axes for larger rotating parts
3. [n]  tree (as opposed to shrub)

ARBOR is a 5 letter word that starts with A.

1. \Ar"bor\, n. [OE. herber, herbere, properly a garden of
   herbs, F. herbier, fr. L. herbarium. See {Herb}, and cf.
   {Herbarium}.]
   A kind of latticework formed of, or covered with, vines,
   branches of trees, or other plants, for shade; a bower. --Sir
   P. Sidney.
   

2. \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.]