The Collaborative International Dictionary of English v.0.48:

Phenomenon \Phe*nom"e*non\, n.; pl. Phenomena. [L.
   phaenomenon, Gr. faino`menon, fr. fai`nesqai to appear,
   fai`nein to show. See Phantom.]
   1. An appearance; anything visible; whatever, in matter or
      spirit, is apparent to, or is apprehended by, observation;
      as, the phenomena of heat, light, or electricity;
      phenomena of imagination or memory.
      [1913 Webster]

            In the phenomena of the material world, and in many
            of the phenomena of mind.             --Stewart.
      [1913 Webster]

   2. That which strikes one as strange, unusual, or
      unaccountable; an extraordinary or very remarkable person,
      thing, or occurrence; as, a musical phenomenon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

phenomenon
    n 1: any state or process known through the senses rather than
         by intuition or reasoning
    2: a remarkable development

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "phenomenon":
   abnormality, actuality, adventure, amazement, anomaly, apparition,
   appearance, astonishing thing, astonishment, circumstance,
   curiosity, episode, event, exception, experience, fact,
   false image, figure, form, gazingstock, hap, happening,
   happenstance, image, incident, marvel, marvelment, matter of fact,
   miracle, mirage, nonesuch, occasion, occurrence, paradox,
   particular, peculiarity, phantom, phasm, portent, presence,
   prodigy, quite a thing, rarity, reality, sensation, shape, sight,
   singularity, something else, spectacle, stunner, turn of events,
   uniqueness, unusualness, vision, wonder, wonderful thing,
   wonderment