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