Source : Webster's Revised Unabridged Dictionary (1913)

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.

            In the phenomena of the material world, and in many
            of the phenomena of mind.             --Stewart.

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

Source : WordNet®

phenomenon
     n 1: any state or process known through the senses rather than by
          intuition or reasoning
     2: a remarkable development
     [also: {phenomena} (pl)]