The Collaborative International Dictionary of English v.0.48:

Heighten \Height"en\ (h[imac]t"'n), v. t. [Written also
   highten.] [imp. & p. p. Heightened; p. pr. & vb. n.
   Heightening.]
   1. To make high; to raise higher; to elevate.
      [1913 Webster]

   2. To carry forward; to advance; to increase; to augment; to
      aggravate; to intensify; to render more conspicuous; --
      used of things, good or bad; as, to heighten beauty; to
      heighten a flavor or a tint. "To heighten our confusion."
      --Addison.
      [1913 Webster]

            An aspect of mystery which was easily heightened to
            the miraculous.                       --Hawthorne.
      [1913 Webster]

WordNet (r) 3.0 (2006):

heightening
    adj 1: reaching a higher intensity; "their heightening fears"