The Collaborative International Dictionary of English v.0.48:

Impressive \Im*press"ive\ ([i^]m*pr[e^]s"[i^]v), a. [Cf. F.
   impressif.]
   1. Making, or tending to make, an impression; having power to
      impress; adapted to excite attention and feeling, to touch
      the sensibilities, or affect the conscience; as, an
      impressive discourse; an impressive scene.
      [1913 Webster]

   2. Capable of being impressed. [Obs.] --Drayton. --
      Im*press"ive*ly, adv. -- Im*press"ive*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

impressiveness
    n 1: splendid or imposing in size or appearance; "the grandness
         of the architecture"; "impressed by the richness of the
         flora" [syn: impressiveness, grandness, magnificence,
         richness]
    2: the quality of making a strong or vivid impression on the
       mind