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):

impressive
    adj 1: making a strong or vivid impression; "an impressive
           ceremony" [ant: unimpressive]
    2: producing a strong effect; "gave an impressive performance as
       Othello"; "a telling gesture" [syn: impressive, telling]

Moby Thesaurus II by Grady Ward, 1.0:

147 Moby Thesaurus words for "impressive":
   Babylonian, Corinthian, absolute, acid, affecting, agitating,
   alive to, arousing, arresting, assuring, august, authoritative,
   aware, awe-inspiring, awesome, awful, barbaric, biting,
   breathtaking, charged, cliff-hanging, cognizant, conclusive,
   conscious, convictional, convincing, corrosive, cutting, decisive,
   deluxe, determinative, disquieting, distracting, disturbing,
   driving, effective, elaborate, electric, elegant, evocative,
   exciting, exhilarating, extravagant, fancy, fine, forceful,
   forcible, formidable, galvanic, glorious, gorgeous, grand,
   grandiose, gutsy, heady, heart-expanding, heart-stirring,
   heart-swelling, heart-thrilling, imperative, imposing, impressible,
   impressionable, incisive, inflammatory, intoxicating, jarring,
   jolting, lavish, luxurious, maddening, magnificent, majestic,
   mind-blowing, mordant, moving, nervous, noble, notable, overcoming,
   overmastering, overpowering, overwhelming, palatial, penetrating,
   perceptive, persuasive, perturbing, piercing, piquant, plush,
   poignant, portentous, posh, powerful, princely, proud, provocative,
   provoking, punchy, ravishing, receptive, redoubtable, ritzy,
   satisfactory, satisfying, sensational, sensible, sensile,
   sensitive to, sentient, sinewed, sinewy, slashing, soul-stirring,
   spirit-stirring, splendacious, splendid, splendiferous, stately,
   stimulating, stimulative, stirring, striking, strong, sumptuous,
   superb, superfancy, superfine, susceptible, susceptive,
   suspenseful, suspensive, swank, swanky, swell, tantalizing,
   telling, thrilling, thrilly, touching, trenchant, troubling,
   unsettling, upsetting, vigorous, vital