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):

impressively
    adv 1: in an impressive manner; "the students progressed
           impressively fast" [syn: impressively, imposingly]
           [ant: unimpressively]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "impressively":
   abundantly, acutely, amazingly, amply, ardently, astonishingly,
   awesomely, conspicuously, copiously, effectively, elaborately,
   elegantly, eloquently, eminently, emphatically, exceptionally,
   expressively, exquisitely, extraordinarily, extravagantly,
   exuberantly, famously, fervently, fluently, forcefully, generously,
   glaringly, glibly, gloriously, glowingly, grandiosely, grandly,
   graphically, imposingly, in glowing terms, incredibly, intensely,
   luxuriously, magically, magnanimously, magnificently, majestically,
   markedly, marvelously, meaningfully, nobly, notably, particularly,
   passionately, peculiarly, pointedly, powerfully, preeminently,
   profusely, prominently, pronouncedly, proudly, remarkably, richly,
   signally, singularly, smoothly, spiritedly, splendaciously,
   splendidly, splendiferously, strikingly, sumptuously, superbly,
   superlatively, surpassingly, surprisingly, tellingly, uncommonly,
   unusually, vehemently, vigorously, vividly, warmly, wonderfully,
   wondrous, worthily