Source : Webster's Revised Unabridged Dictionary (1913)

Sporadic \Spo*rad"ic\, a. [Gr. ? scattered, fr. ?, ?, scattered,
   fr. ? to sow seed, to scatter like seed: cf. F. sporadique.
   See {Spore}.]
   Occuring singly, or apart from other things of the same kind,
   or in scattered instances; separate; single; as, a sporadic
   fireball; a sporadic case of disease; a sporadic example of a
   flower.

   {Sporadic disease} (Med.), a disease which occurs in single
      and scattered cases. See the Note under {Endemic}, a.

Source : WordNet®

sporadic
     adj : recurring in scattered and irregular or unpredictable
           instances; "a city subjected to sporadic bombing raids"
           [ant: {continual}]