The Collaborative International Dictionary of English v.0.48:

Thuja \Thu"ja\ (th[=u]"j[.a]), n. [NL., from Gr. qyi`a an
   African tree with sweet-smelling wood.] (Bot.)
   A genus of evergreen trees, thickly branched, remarkable for
   the distichous arrangement of their branches, and having
   scalelike, closely imbricated, or compressed leaves. [Written
   also thuya.] See Thyine wood.
   [1913 Webster]

   Note: Thuja occidentalis is the Arbor vitae of the
         Eastern and Northern United States. Thuja gigantea of
         North-western America is a very large tree, there
         called red cedar, and canoe cedar, and furnishes a
         useful timber.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Arbor vitae \Ar"bor vi"t[ae]\ [L., tree of life.]
   1. (Bot.) An evergreen tree of the cypress tribe, genus
      Thuja. The American species is the Thuja occidentalis.
      It is commonly used as an ornamental shrub around homes.
      [1913 Webster +PJC]

   2. (Anat.) The treelike disposition of the gray and white
      nerve tissues in the cerebellum, as seen in a vertical
      section.
      [1913 Webster]